WASHINGTON, Sept. 27 — The Food and Drug Administration does very little to ensure the safety of the millions of people who participate in clinical trials, a federal investigator has found.

In a report due to be released Friday, the inspector general of the Department of Health and Human Services, Daniel R. Levinson, said federal health officials did not know how many clinical trials were being conducted, audited fewer than 1 percent of the testing sites and, on the rare occasions when inspectors did appear, generally showed up long after the tests had been completed.

The F.D.A. has 200 inspectors, some of whom audit clinical trials part time, to police an estimated 350,000 testing sites. Even when those inspectors found serious problems in human trials, top drug officials in Washington downgraded their findings 68 percent of the time, the report found. Among the remaining cases, the agency almost never followed up with inspections to determine whether the corrective actions that the agency demanded had occurred, the report found.

“In many ways, rats and mice get greater protection as research subjects in the United States than do humans,” said Arthur L. Caplan, chairman of the department of medical ethics at the University of Pennsylvania.