Monoclonal antibodies to progesterone 11α-hydroxy hemisuccinate were prepared. Supernatant of clone hybridoma reacted positively to the progesterone solution. Dotblot analysis revealed a single precipitation band when rabbit anti-mouse IgM and anti-mouse κ were used. These monoclonal antibodies did not react with pregnenolone, testosterone, estrone, estradiol-17β, aldosterone, hydrocortisone (cortisol), cortisone, corticosterone and 11α-dehydroxycortisone (DOC). Radioimmunoassay using those monoclonal antibodies was developed. Data from this assay supported the following conclusions: (1) association constant (K_A) of supernatant of clone PrM for progesterone was 2×10¹⁰, (2) monoclonal antibodies reacted to progesterone most efficiently at 1 : 10,000 dilution, and (3) efficiency of reactivity of monoclonal antibodies was higher than that of rabbit antibodies.