The H-Y (male) antigen is detectable on 50% of eightcell mouse embryos. We show that 82% of the offspring resulting from the transfer of (C57BL/6XA/J)F1embryos surviving treatment with H-Y antibody and complement to the uteruses of pseudopregnant mice are female. The increased ratio of female-to-male births indicates that the H-Y-bearing embryos are male.