Sex reversed X/X male mice carrying Sxr or the variant Sxr' were typed for expression of the male specific histocompatibility antigen H-Y, by skin grafting and by in vitro cytotoxic and proliferative tests. The X/XSxr males, like X/Y males, were H-Y positive by in vitro testing, and failed to reject semi-syngeneic male skin grafts. In contrast X/XSxr' males, like X/X females, were H-Y negative and rejected semi-syngeneic, male skin. Spleen cells from X/Y males sensitized C57BL/10 female recipients to reject syngeneic male skin rapidly, whilst immunization with X/X female or X/XSxr' male cells failed to stimulate such second-set responses. These data suggest that the H-Y antigen detected by cytotoxic T cells is the same as that detected by graft rejection responses, that the Sxr' variant is not a tissue-specific regulatory mutation, and that X/XSxr' individuals do not express H-Y antigen but nevertheless develop as phenotypic males.