A specific, sensitive, precise and accurate radioimmunoassay has been developed for fluoxymesterone, 9alpha-fluoro-11beta,17beta-dihydroxy-17-methyl-4-androsten-3-one (Halotestin). The method is capable of detecting 25 picograms of drug in 0.1 ml of unextracted serum. The primary antibody was prepared against fluoxymesterone 3-(0-(carboxy-methoxime)) (CMO) bovine serum albumin. The specificity of the assay is greatly influenced by the hydroxyl group at position 11 and the methyl group at position 17. Physiological levels of endogenous steroids did not cross-react significantly with the primary antibody. Blood levels of fluoxymesterone were determined in both human subjects and male beagle dogs after oral administration of Halotestin.