Edward Adelbert Doisy, Sr. (1893-1986) was awarded half of the 1943 Nobel Prize in Physiology or Medicine for his discovery of the chemical nature of vitamin K. He successfully isolated the vitamin, which had been previously found in alfalfa. Doisy studied analogues of vitamin K and established the distinction between vitamin K1, isolated from alfalfa, and vitamin K2, isolated from fish meal. He also synthesized vitamin K in 1939. In addition to his work on vitamin K, Doisy also conducted biochemical studies on sex hormones. Along with Edgar Allen, he refined the vaginal cytology (or smear) technique to examine the potency of estrogenic hormones in ovariectomized rats (rats with surgically-removed ovaries). He also improved methods used to isolate and identify insulin and contributed to work on antibiotics, blood buffer systems, and bile acid metabolism.