A specific progesterone antibody was obtained by immunizing a ewe with 11α-hydroxy-progesterone-hemisuccinate conjugated to bovine serum albumin. This antibody was used for the assay of progesterone in crude plasma extracts. The sensitivity of the assay was 0.05 ng. The accuracy was confirmed by the recovery of known amounts of progesterone added to plasma. The inter- and intra-assay coefficient of variation was 8%. Specificity was confirmed by a) checking cross reactivity of various steroids; b) failure of other steroids added to plasma to influence the accurate measurement of progesterone, and c) finding comparable progesterone levels in plasma extracts with and without chromatography. Mean values of progesterone (ng/ml) were 0.26 ± 0.08 (sd) in males and postmenopausal females; 0.50 ± 0.44 in preovulatory females; and 12.1 ± 4.81 in postovulatory females.