The question is raised whether the H-Y transplantation antigen, as defined by skin graft rejection, is the same as the “male-specific” antigen that is identified by H-Y antibodies. We have addressed that question by injecting female mice of the inbred C57BL/6 strain with serologically defined H-Y antigen, and measuring the response of those female mice to skin grafts from intrastrain male mice. The results of our study indicate that the “two” male antigens are crossreactive or identical.