The radioimmunoassay described for the determination of progesterone only involves extraction of the plasma andsubsequent performance of the protein-binding reaction. The antisera used were obtained from rabbits and directed against 11α -OH-progesterone hemisuccinate BSA. A high degree of reliability was obtained for the criteria specificity, sensitivity, accuracy and precision. Application of the method during the bovine oestrous cycle revealed that there is no preovulatory rise of progesterone in this species. The presence of progesterone in peripheral plasma of bulls was demonstrated.