A small but groundbreaking study of infants who received vaccines containing a mercury-based preservative has found the levels of mercury in their blood were well within the federal safety limits.

The study, reported Saturday in The Lancet, a London-based medical journal, also found that infants excrete the mercury much faster than expected, suggesting it does not build up from one vaccination to the next.

The preservative, thimerosal, is no longer used in American vaccines for infants under 6 months old, but the issue is important to parents of autistic children who have filed hundreds of damage claims and lawsuits against thimerosal's maker. A clause protecting Eli Lilly & Co., the manufacturer, from lawsuits was added to the domestic security law signed by President Bush Nov. 25.

The director of the Institute for Vaccine Safety at Johns Hopkins University, Dr. Neal Halsey, praised the study as "much needed and done quite well," although more work needs to be done.

But Sallie Bernard, director of Safe Minds, a parents group suing the vaccine industry, vehemently attacked it, calling its optimistic conclusions "very much off-base."

Mercury is unquestionably poisonous. At extreme doses, it causes tremors and madness. Children who accidentally get high doses tend to speak and walk later and have tics and lower intelligence, but not autism, medical experts say.

Small amounts, however, are common in soil and plants, in power plant fumes and in dental fillings. Fish are the largest source for humans, and a tuna sandwich may contain more mercury than a vaccine shot.

No study has proved that thimerosal causes any ill effects, but at the urging of federal health officials, vaccine makers began eliminating it in mid-1999. The study began with that recommendation.

Thimerosal, which kills funguses and bacteria, is still used to preserve vaccines sent to the Third World, and the World Health Organization defends it. The vaccines prevent common diseases there, so the benefits far exceed potential side effects.

The Lancet study, led by Dr. Michael Pichichero of the University of Rochester, tested the blood, urine and stool of 33 infants ages 2 months to 6 months, all of them seen by Rochester, N.Y., pediatricians injecting thimerosal-containing vaccines. They were compared with 15 infants seen at a clinic in Bethesda, Md., using mercury-free vaccines.

In their first six months, children typically receive three doses of diphtheria-tetanus-pertussis vaccine, one or two for hepatitis B, and sometimes up to three for haemophilus influenza. Other vaccines, like polio, may not contain thimerosal.

The Environmental Protection Agency's safe level for mercury in children's blood is 5.9 parts per billion. That, Pichichero explained, is based on a study of children in the Faroe Islands, south of Iceland, whose mothers ate whale blubber polluted with mercury and PCB's. When the mothers had 59 or more parts per billion of mercury in their blood while pregnant, their children scored lower on intelligence tests several years later. The EPA took one-tenth of that -- 5.9 parts -- as a safe level.

All but one of the infants in the group exposed to thimerosal had blood levels of 1 to 3 parts per billion; the one exception went to 4.1. In the mercury-free control group, only one baby had even a measurable level of mercury..