n the Sacramento City Unified School District in California, Inger Lindholm is affectionately known as the Lice Queen.

A nurse for more than 40 years, she is highly skilled at detecting lice creeping through a child's scalp and even better at the difficult task of spotting the eggs, or nits, that the bugs leave behind on the hair shafts.

If Ms. Lindholm finds a louse or an egg, the child is sent home until the lice are dead and eggs are removed. If she learns that two or more children in a class have lice or eggs, she inspects the entire class head by head, hair by hair.

"I try not to make the screening a negative experience," Ms. Lindholm, 65, said.

The Sacramento district has a no-nit policy, meaning that children have to be excluded from school until all head lice and nits have been removed. Though rules differ, the National Association of School Nurses estimates that as many as 90 percent of all United States schools have one type of policy or another.

Although children can be kept out of school for the presence of lice or nits, few agree how to treat the problem. Chaos, confusion and a fair amount of anger abound.

Some research suggests that lice and their eggs are increasingly resistant to the over-the-counter shampoos that most pediatricians and school administrators recommend. A study in the current issue of The Archives of Dermatology adds to a growing body of evidence that resistance to over-the- counter treatments to kill lice "is reaching chronic levels in the U.S."

With no universally agreed on method of killing lice and their eggs, desperate parents are trying mayonnaise, mouthwash and even hiring "nit pickers" to make house calls. Anti-louse entrepreneurs have posted their wares on the Web, and some have recently been scolded by the Federal Trade Commission for exaggerated claims.

"The only completely guaranteed method to make sure your child is free of head lice is to give your kid the Telly Savalas look and shave his or her head," said Dr. Richard Pollack, an instructor in the department of immunology and infectious diseases at the Harvard School of Public Health.

Statistics about the prevalence of head lice are generally unreliable because reporting cases is not mandatory. But estimates range from 6 million to 12 million cases a year.

What is known is that a head louse is a blood-sucking insect about the size of a sesame seed that feeds, mates and lays its eggs on the human head. The eggs are coated with a gluelike substance that adheres them to the hair shaft.

Contrary to a widely held belief, head lice are not linked to poor hygiene. In fact, they may be more common in children with clean hair and good grooming habits. Head lice are most often found in children from prekindergarten to third grade who spread them by direct contact or with objects like hats or brushes. The lice cannot jump from person to person.

Girls are four times as likely as boys to have lice, probably because they are more apt to have long hair, and their play involves more physical contact.

Several studies have shown that head lice have grown more difficult to kill. Over the last 10 years, researchers from the Czech Republic, England, France and Israel have pointed to increased resistance to permethrin, the active ingredient in anti-lice shampoos most often recommended by physicians.

In a study published in 1999 in The Archives of Pediatric and Adolescent Medicine, Dr. Pollack and his colleagues at Harvard found that head lice collected from two sites in the United States were not killed by permethrin, even when exposed to progressively higher doses. In contrast, lice collected from children in Borneo, where permethrin is rarely used, were killed.

Most recently, Terri L. Meinking, a research assistant professor at the University of Miami School of Medicine and a well- known lice expert, looked at two prescription medications and several over-the- counter formulas to find out whether lice had become resistant to them.

Published in February in The Archives of Dermatology, her study found that Nix killed 3 percent to 5 percent of lice after 10 minutes (the recommended time) and 8 percent to 10 percent after 20 minutes.

RID, the market leader, killed 7 percent in 10 minutes. A-200, which is similar to the formulation of Nix, was more effective. A prescription medication, Ovide, containing malathion, was the most effective, killing 88 percent of lice in 10 minutes.

The results were compared to a study by Professor Meinking in 1986 that found that all the products except Ovide failed to work as well as they had previously.

"Lice have genetically mutated to survive the medication," said Professor Meinking, who has had to battle head lice on her own daughter. "They can walk around on permethrin and not be fazed. It's clear that these products don't work."

Dr. Arthur Ciociola, vice president for consumer health care research and development at Pfizer, the maker of Nix, said he had not seen the results of Professor Meinking's study. But he pointed out that the Food and Drug Administration continued to approve the sale of Nix for killing head lice.

"Parents should feel confident," Dr. Ciociola said, "that Nix remains an effective and safe treatment for head lice when label instructions are carefully followed."

Dr. Andrew M. Snoddy, senior associate director of clinical research at Bayer, the maker of RID, argued that the results of the study were not valid because it was not conducted on an actual infestation of head lice.

"No conclusions about the effectiveness of RID can be drawn from this study because RID's directions for use were completely ignored," said Dr. Snoddy.

Despite questions about resistance, the over-the-counter shampoos remain the most commonly recommended form of treatment. The most recent guidelines on eradicating lice were published in the August 2000 issue of the journal Contemporary Pediatrics. As first-line treatment, it recommends using an over-the-counter shampoo and a metal lice comb and a second treatment eight to 10 days later.

If live lice are present after those two treatments, the journal suggests using prescription medicine with malathion combined with combing to remove the nits.

But two parent groups, the National Pediculosis Association based in Newton, Mass., and the American Head Lice Information and Resource Center in Cambridge, criticize the guidelines.

"There are legitimate scientific reports telling us that these products aren't working," said Deborah Z. Altschuler of the N.P.A., which sells its own lice removal products. It is also unclear whether nits in the hair signal an actual infestation of lice. Experts note that nits can be difficult to identify, and dandruff or dried hair spray have been mistaken for them.

Plus, studies show that a large proportion of nits never become lice. A study by the Centers for Disease Control and Prevention published last year in the journal Pediatrics found that of 50 children with just nits, 9 children (18 percent) became infested with lice in the two-week follow-up.

"Most nits in the hair are empty shells," said Dr. Keoki Williams, the lead author of the study, who is now an internist for the Henry Ford Hospital in Detroit. "Children with nits but without an active infestation should simply be watched very closely, rather than removed from school."

Lisa Finch of Henderson, N.C., said her 6- year-old recently missed two weeks of school after having been repeatedly sent home with nits. After trying several over- the-counter products, as well as combing the girl's waist-length hair, a pediatrician found the problem was dandruff.

Ms. Finch said she was working to overturn her district's no-nit policy.

Other parents and school officials are considering similar action. Last month, the Cherry Hill, N.J., school district abolished its no-nit rule amid much controversy.

Recently, the school nurses' association and the National Academy of Pediatrics issued statements against the policies.

Still, many parents and nurses want to keep the no-nit policies, to prevent children from being infected or reinfected. "I don't know what we'd do without it," said Jill Reynolds, a certified school nurse in Kalamazoo, Mich. "Our policy is intended to keep children in school. What kind of quality control do you have if a lice-free, nit-free environment isn't being maintained?"

As the debate continues, some parents have turned to professional "nit pickers." Several years ago, Dr. Robert Schachter, a psychologist by training, started Licenders in New York with a team of "nit pickers" that removes lice and eggs for $65 an hour. "Lice," Dr. Schachter said, "are a big inconvenience and very emotionally stressful."