Testosterone-3-O-carboxymethyl-oxime derivative was synthetized and coupled to bovine serum albumin (BSA). The T-3-CMO-BSA conjugate homogenized with Freund's adjuvant used as immunogen was injected multiple sites in rabbits. The antisera collected were characterized in a radioimmunological system, separation with dextran-charcoal using 125I-Testosterone as tracer. The antibody titres varied from one animal to another. The titre of anti-T serum selected for RIA was 1: 10(4)-1: 2 X 10(4) (initial dilution). All anti-T sera 100 percent crossreacted with 5 alpha-dihydrotestosterone but nonsignificant interference was observed with other C19, C21 and C18 steroids. The affinity constant of the selected anti-T serum was in the range 1.4-1.9 X 10(9) litres/mole. The data so far published on the antisera toward testosterone are reviewed. We conclude that the selected anti-T-3-CMO-BSA serum may provide assays for testosterone with potential for clinical applications.