Investigating the interference of danazol in testosterone radioimmunoassays, we measured serum testosterone concentrations in the serum of women receiving danazol, with three testosterone radioimmunoassay kits. All of these showed positive interference by danazol or its metabolites (or both). Values for testosterone obtained with two of the methods (Diagnostic Products Corp. and Farmos Diagnostica) depended on the volume of serum extracted per assay tube. A detailed column-chromatographic method is presented in which testosterone is resolved from danazol and its metabolites.