This experiment was carried out to develop a new technique of identifying XX bearing embryos prior to implantation by immunological method. H-Y antiserum was prepared in inbred Wistar female rats by repeated immunization with new-born testis supernatant and spleen cells from males of the same strain. The activity of H-Y antibody in antiserum was tested by ELISA, and biological tests. The results obtained in this experiment are follows: The formation of H-Y antibody in rats was detected by ELISA test and biological test. The optimal density(O.D.) value of the H-Y antiserum of the female rats immunized with spleen cells. new-born testis or the compound of both antigens was in the range of 0.12 to 0.25, so that H-Y antibody was ascertained to be formed. The biological test with the 8 to 16 cell stage embryos showed that H-Y antibody was formed in all female rats immunized with spleen cells, but that it was formed only in 80% female rats immunized with the other antigens. Of 160 morulas treated with H-Y antiserum and complement, 84(52.5%) embryos were developed to blastocyst and 76(47.5%) embryos were affected significantly to lysis(p $lt;0.001). The media treated with D-PBS, D-PBS + NGPS, D-PBS + NGPS + normal rat serum or D-PBS + H-Y antiserum affected significantly 2(3.0%), 1(2.3%), 2(5.8%) and 4(8.7%) embryos to lysis, respectively(p $lt;0.001).