ALTIMORE, July 23 — A federal agency investigating the death of a volunteer in a research study at Johns Hopkins University announced today that it was easing its four-day- old suspension of research experiments involving human subjects at the university.

But because of strict conditions imposed by the agency, the Office for Human Research Protections, most of the thousands of studies that were suspended are unlikely to resume for weeks or months as they undergo comprehensive new reviews by panels at the university and officials at the agency.

Still, some studies involving either minimal risks to subjects or treatments of gravely ill patients — for example, those taking experimental cancer drugs — may go forward almost immediately, the agency said.

Although some of those exemptions already existed, Hopkins officials generally praised the decision.

"We're extremely glad this has been lifted," said Dr. Chi Van Dang, vice dean for research at Hopkins.

University officials had excoriated the agency for imposing the suspension last Thursday, calling that decision outrageous and excessive. Today, Dr. Dang said he hoped that negotiations would succeed in further loosening the restrictions.

"There should be additional dialogue," Dr. Dang said.

Despite the quick turnaround, Dr. Arthur Caplan, an ethicist at the University of Pennsylvania, said that the episode at Johns Hopkins, which followed recent suspensions of human research at Duke University, Penn and other prestigious institutions, suggested that the problems were widespread.

"There are core problems with human subject protections in this country," Dr. Caplan said. "Having Hopkins, if you will, back online doesn't get at those."

In a visit to Johns Hopkins last week, agency officials determined that the university panels that consider proposals for human research were overburdened, understaffed and lacking in a detailed understanding of federal regulations governing human research. The panels, called institutional review boards, are usually composed of people and researchers inside and outside the university.

In responding to those concerns, Johns Hopkins officials worked intensively over the weekend with the Office for Human Research Protections on a plan to improve the way in which the university assesses its scientists' proposals to conduct research involving humans. The agency conveyed its decision to ease the suspension in a letter to the officials that was dated July 22.

The Office for Human Research Protections "finds that your institutions have developed a satisfactory corrective action plan to address all areas of noncompliance and concerns," said the letter, which was signed by Dr. Michael Carome, director of the division of compliance oversight at the agency.

The letter said the reinstatement was immediate.

Gary Stephenson, a Hopkins spokesman, said about 2,400 trials involving thousands of volunteers had been suspended by the earlier order. The university believes that only several hundred of those trials will resume without new review, Mr. Stephenson said..

In addition to their negotiations with the research protections office, Hopkins officials found advocates in Maryland's two United States senators, Barbara A. Mikulski and Paul S. Sarbanes, both Democrats. They sent Secretary of Health and Human Services Tommy G. Thompson a letter urging him to consider the university's concerns.

But today a spokesman for the health department, which is responsible for the oversight agency, said that while Mr. Thompson had been kept informed of the weekend deliberations, there had been "no political pressure whatsoever" to lift the suspension.

"The secretary understands the importance of human subject protections, and the importance of ensuring that institutions do all they can to protect human subjects," said the spokesman, William Hall. "O.H.R.P. has been left to do the job they do."

In a statement today, Senator Mikulski said her actions had been intended to "ensure that the needs of the patients were being addressed and that they were not left in limbo."

The case began when Ellen Roche, a young, healthy technician working at the Johns Hopkins Asthma and Allergy Center, volunteered to take part in a study to explore the causes of asthma that was being undertaken by Dr. Alkis Togias, an associate professor of immunology at the university. Ms. Roche, 24, did not work directly for Dr. Togias, but within a separate research group there.

On May 4, as part of the study, Ms. Roche inhaled a gram of an unapproved drug, hexamethonium, and within days developed a fever and flu-like symptoms. After her lungs gradually deteriorated and her kidneys failed, apparently because of the drug, she died on June 2. Hopkins announced her death in mid-June.

Internal reviews at Hopkins found that Dr. Togias had made several missteps in the project, including a failure to turn up literature from the 1950's indicating that hexamethonium can be toxic to lungs and not informing the university's review panels, called institutional review boards, that an earlier subject in the same experiment had developed a cough and shortness of breath. All of Dr. Togias's research remains suspended.

But last week, after its visit to Johns Hopkins, the agency determined that there were widespread problems with the way the review boards conducted their business, Mr. Hall said.

The agency, Mr. Hall said, "determined that because of these systemic problems there are concerns about the potential risks to other potential volunteers."

In essence, the agency determined that the review boards did not consider each individual proposal carefully enough, that they were overburdened and understaffed, and that review board members often did not understand the federal regulations governing experiments involving humans.

Sunday's letter from the agency said the university had come up with a plan to shore up all of those problems. For example, Mr. Hall said, Hopkins will increase the number of review boards to four, from three, and consider each application more carefully.

Research involving minimal risk to subjects — procedures like saliva swabs and urine tests — can proceed almost immediately. Other projects, involving the "best interests of the patient," like treatment for severe illnesses, can also go ahead at the discretion of the university, he said.

All others must be reviewed again according to the new procedures, and monthly progress reports must be sent to the agency.

"We would anticipate that it would take weeks if not months," Mr. Stephenson said.

Meanwhile, the larger issue of protecting human subjects remains, said Dr. Ernest D. Prentice, associate vice chancellor for academic affairs at the University of Nebraska Medical Center and a consultant to the oversight agency.

"We have a fair amount of evidence to indicate that there is probably a systemic problem in the U.S. relative to the adequacy of human research protection," Dr. Prentice said, "and that leading institutions are not immune."