1. The raising of antibodies in Soay sheep to an immunogenic conjugate of 19-nortestosterone-3-(O-carboxymethyl)oxime with bovine serum albumin is described. 2. High titre plasma so obtained can be used for the radioimmunoassay of 19-nortestosterone. 3. Cross-reactivities of the antibody with endogenous steroids is confined to testosterone (20-25%) and 17beta-estradiol (1-2%). Cross-reactivities with a range of other 17beta-hydroxy synthetic anabolic steroids and with some potential 17beta-hydroxy metabolites of 19-nortestosterone vary between 2-3% for 5beta-estrane-3alpha,17beta-diol and 26% for 1-dehydrotestosterone. There is limited cross-reactivity (ca. 1-2%) with anabolic steroids with 17beta-hydroxy, 17alpha-methyl substituents. 4. 19-Nortestosterone (phenyl propionate) administered to a horse (1 mg/kg, intra-muscular) gives urinary metabolites readily detectable by radioimmunoassay, for more than 4 days.

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