Histocompatibility-2 antigens were detected on preimplantation mouse embryos of the 8-cell and blastocyst stages of development by an embryo cytotoxicity assay in which anti-H-2 serum and complement were incubated with embryos and cell death was measured by inhibition of ('3)H-thymidine incorporation into DNA. It was found that H-2 antigens on blastocyst embryos exhibit specificities which differ from those specificities present on adult lymphocytes. This general multispecific H-2 antigen on the embryo was postulated to serve the function of allowing the embryo to escape immune surveillance by the mother during early mammalian embryogenesis;Ia antigens were not detectable on blastocyst embryos and this finding confirms earlier, negative reports. (beta)(,2)-microglobulin was not detected on mouse blastocysts which expressed H-2 antigens. A cell-mediated lympholysis reaction, in which cytotoxic T cells were used against blastocyst embryo targets, showed the functional presence of H-2 antigens on blastocyst embryos;The ability of preimplantation mouse embryos to incorporate ('3)H-thymidine into their DNA was used to examine the effects of salt, (alpha)-amanitin, aphidicolin, theophylline, cycloheximide, puromycin, and hCG on cell division during early embryogenesis. Conditions for the DNA synthesis assay were optimized as follows: (1) 4(mu)Ci/ml ('3)H-thymidine (sp. act. 20 Ci/ml); (2) a ('3)H-thymidine labeling period of two to seven hours; (3) a three hour preincubation period for blastocyst embryos; (4) from 1-64 embryos per assay; and (5) presence or absence of the zona pellucida. Salt, (alpha)-amanitin, aphidicolin, cycloheximide, and puromycin were able to inhibit cell division, whereas theophylline and hCG had no effect on cell division during early embryogenesis.