The expression of H‐Y antigen on preimplantation mouse embryos has been studied by complement dependent cytotoxicity. Embryos at the 8‐cell stage were exposed to anti‐H‐Y and complement and then scored for cell death. Overall, half of the treated embryos were killed, retarded, or contained dead cells. Of those embryos which were not affected by anti‐H‐Y, 92% were female. In control experiments, neither normal serum plus complement nor complement alone had any specific effect on male embryos. It is concluded, therefore, that the H‐Y antigen is present on preimplantation