A two step-method was applied for the preparation of a tracer adequate for a radioimmunoassay (RIA) system for testosterone. Histamine was radioiodinated by the Chloramine-T method and then coupled to testosterone-3-carboxymethyl-oxime (T-3-CMO) derivative. After purification by TLC, the steroid tracer was stable in ethanol for at least four months. Using an anti-T-3-CMO-BSA serum, a (bridge) homologous RIA system for testosterone was developed. The reagents (antiserum, tracer and standard) were incubated 2 hrs at 37 degrees C and then the free radioactivity was removed by a dextran-charcoal suspension. The testosterone RIA system, with the sensitivity of 30 pg/tube, is suitable for the measurement of the steroid hormone in biological fluids and tissues.