Eleven transexuals with transexuality from man to woman and a karotype 46 X-Y and 11 transexuals with transexuality from woman to man and the karotype 46 X-X were investigated for H-Y antigen. Eight of the 46 X-Y transexuals were H-Y antigen negative, one intermediately slightly positive and two H-Y antigen positive. Of the 46 X-X transexuals 9 were H-Y antigen positive, one was intermediately slightly positive and one was H-Y antigen negative. Two groups of transexuals can be distinguished. Genuine transexuals and secondary transexuals with a transvestite past who have suffered a sexual identity crisis due to the environment. The H-Y antigen is an expression of a complex of the genes which is responsible for virilization and is likely located in the Y chromosone. The gene responsible for the H-Y antigen expression is not identical with the inductor for testicular development. An explanation for the disordance of the H-Y antigen findings in transexuals is translocation or gene exchange from a Y-chromosome to an X-chromosome during meisis of the spermatogonia. In this way a morphological substrate for transexuality has been detected. H-Y antigen negative 46 X-Y males and H-Y antigen positive 46 X-X females can now be diagnosed as genuine morphological transexuals.