The plasma from 21 animals was examined for a testosterone-binding protein (TeBG) with polyacrylamide gel electrophoresis. This technique allows separation of TeBG from other steroid-binding proteins. TeBG was identified in the plasma of only 13 of 21 species examined, indicating that this protein has a more limited species distribution than cortisol-binding globulin. Estradiol binding to TeBG was demonstrated in five species. The influence of TeBG on androgen metabolism is discussed.