Abstract The need for a simple and fast quantitative method for progesterone with routine laboratory and clinical application motivated the development of a radioimmunoassay. Progesterone antibodies were produced in rabbits by immunization with a bovine serum albumin conjugate of 11α-hydroxyprogesterone hemisuccinate. The antiserum was purified with rivanol and used at a 1:480 final dilution. The assay procedure was based on the ability of unlabeled progesterone to displace antibody-bound tritiated progesterone. Linearity existed between 0.25 and 4 ng of progesterone, and 2.21 ng displaced 50% of the bound label. The assay detected as little as 0.25 ng of progesterone. Furthermore, the index of precision was calculated to be 0.146. Examination of the binding affinities of a wide variety of naturally occurring steroids demonstrated that the antibody was highly specific for progesterone. Using ether extraction of saline or monkey plasma containing known amounts of progesterone, recoveries were found to approximate 100%. Employing this procedure, progesterone was measured in 0.5 ml aliquots of female rhesus monkey plasma taken during selected days of the menstrual cycle, and the results obtained were in agreement with literature values.