Genes associated with the H-2 complex were shown to control the timing of early development. The gene(s) responsible for the effects was named Ped for preimplantation embryo development. Using congenic mice, it was shown that "slow" development is associated with the H-2('k) haplotype. The gene(s) shows its effect as early as the first cleavage division, but not as early as the time of ovulation. The Ped gene(s) was shown to control not only the timing of the first cleavage division but the subsequent rate of development as well. Genetic analysis of F(,1) embryos showed dominant expression of the gene(s) and no maternal effects. Analysis of F(,2) and backcross embryos showed that background genes and environment are also involved in control of the timing of early development;With the purpose of searching for the Ped gene products, monoclonal antibodies were produced against H-2 antigens. Using a cytotoxicity assay previously developed in the laboratory and monoclonal ascites fluid, it was shown that early embryos express H-2 antigens. It was observed that expression can vary among strains. This supports the theory that the Ped product(s) could be the H-2 antigens themselves;This is the first description of H-2 associated genes involved in the control of early development. A quantitative genetics interpretation of the results is also included.