Antisera to histocompatibility (H-Y) antigen were used to immunologically presume the sex of rabbit embryos. H-Y antisera were prepared in inbred SD female rat by repeated immunization of spleen cells from males of same strain. The titre of H-Y antibody in antiserum was examined by mouse sperm cytotoxicity and biological tests. Experiments applied delaying ability of development of embryos in H-Y antiserum and binding ability of FITC labelled second antibody. After culture, embryos were observed their morphological characteristics under phase contrast microscope and detected fluorescence on embryos under fluorescence microscope. After detection of fluorescence, embryos were transfered to normal medium and observed their morphological characteristics. 1. When rabbit morula were treated with H-Y antiserum only, the rate of developed and delayed embryos was 47.2 and 52.8% respectively, and the rate of non-fluorescing and fluorescing embryos was 51.4 and 48.6%, respectively. 2. When rabbit morula were cultured in H-Y antiserum followed by complement, the rate of non-fluorescing and fluorescing embryos was 53.6 and 46.4%. respectively. 3. After detection of fluorescence, the embryos were cultured in normal medium. When embryos were treated with H-Y antiserum only, the rate of arrested and developed embryos was 20.8 and 79.2% respectively. However, when embryos were treated with H-Y antiserum followed by complement, the rate of arrested and developed embryos was 42.9 and 57.1% respectively.