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An antiserum against turosteride (code name FCE 26073), a potent testosterone 5 alpha-reductase inhibitor, has been raised in rabbits by immunization with an immunogen produced by conjugation of a derivative of FCE 26073 (FCE 27424) to bovine serum albumin. The antiserum was able to distinguish FCE 26073 from its derivatives modified at the 17 beta position and from all the endogenous steroids tested. A radioimmunoassay for the determination of FCE 26073 in human plasma and urine was developed using this antiserum and tritium labeled turosteride. FCE 26073 was extracted from 50 microliters of plasma or 25 microliters of urine using ethyl-ether with a recovery greater than 90%. Using this procedure it was possible to achieve a final limit of quantitation of 142 pg/ml in plasma and 284 pg/ml in urine. The assay was validated in terms of reproducibility, accuracy and precision in the range 3.9-250 pg/50 microliters of plasma and 25 microliters of urine. The plasma concentration of FCE 26073 in a healthy male volunteer who received 0.2 mg of the drug was measured using the radioimmunoassay.