American scientist, best known for her contributions to the tuberculin test and to safety measures for intravenous drug therapy.

Seibert contracted polio at age three, but became an outstanding student, graduating at the top of her high-school class and winning a scholarship to Goucher College, Towson, Maryland, from which she graduated in 1918. She worked during World War I as a chemist at Hammersley Paper Mill and then won a scholarship to Yale University in New Haven, Connecticut, where she earned a doctorate in biochemistry in 1923. When, in the course of her doctoral research, she discovered that intravenous injections made with contaminated distilled water could cause fevers in patients, she invented a new distillation process that eliminated all bacteria. After doing postdoctoral research at the University of Chicago, Seibert taught pathology at the Sprague Memorial Institute in Chicago and biochemistry at the University of Pennsylvania's Henry Phipps Institute.

In 1937–38, as a Guggenheim fellow at the University of Uppsala, Sweden, she invented a means of isolating and purifying the active substance in tuberculin (the protein substance from the tuberculosis-causing bacillus Mycobacterium tuberculosis). In producing the first purified protein derivative tuberculin, Seibert enabled the first reliable tuberculin test. Adopted as the standard by the United States in 1941 and by the World Health Organization in 1952, this skin test for tuberculosis is still in use today.

Seibert retired from teaching in 1958 and served as a consultant to the United States Public Health Service and then as director of the Cancer Research Laboratory at the Mound Park Hospital (now Bayfront Medical Center), St. Petersburg. She remained active in experimental cancer research until failing health and complications from her childhood polio forced her to stop working. In 1990 Seibert was inducted into the National Women's Hall of Fame.