The sedative was Kevadon, and the application to market it in America reached the new medical officer at the Food and Drug Administration in September 1960. The drug had already been sold to pregnant women in Europe for morning sickness, and the application seemed routine, ready for the rubber stamp.

But some data on the drug’s safety troubled Dr. Frances Oldham Kelsey, a former family doctor and teacher in South Dakota who had just taken the F.D.A. job in Washington, reviewing requests to license new drugs. She asked the manufacturer, the William S. Merrell Company of Cincinnati, for more information.

Thus began a fateful test of wills. Merrell responded. Dr. Kelsey wanted more. Merrell complained to Dr. Kelsey’s bosses, calling her a petty bureaucrat. She persisted. On it went. But by late 1961, the terrible evidence was pouring in. The drug — better known by its generic name, thalidomide — was causing thousands of babies in Europe, Britain, Canada and the Middle East to be born with flipperlike arms and legs and other defects.

Dr. Kelsey, who died on Friday at the age of 101, became a 20th-century American heroine for her role in the thalidomide case, celebrated not only for her vigilance, which spared the United States from widespread birth deformities, but also for giving rise to modern laws regulating pharmaceuticals.