Ein3radioimmunoassay (RIA) for precise and reliable determination of androstenedione (androst-4-ene-S,17-dione, AD) in endocrinological investigations has been developed. Specific antisera were obtained from rabbits following immunization with the bovine serum albumine conjugate of AD-3- (O-carboxymethyl)oxime. The sensitivity of the method was found to be 18fmol AD per assay tube. The method described has proved to be suitable for determinations of AD values in blood plasma and follicular fluids of cattle and swines.