Mouse embryos at the 2‐cell, 8‐cell, and blastocyst stages of development were examined for the presence of H‐2 antigens by immunoperoxidase labeling and transmission electron microscopy. Conventional antisera made in congenic mouse strains were used to study embryos of four different haplotypes: b, a, k, and d. Blastocysts showed uniform heavy labeling of all cells of the trophectoderm, 8‐cell embryos showed lighter labeling of only some of the cells, and 2‐cell embryos showed no labeling. Similar results were found for all four haplotypes studied. In addition, monoclonal antibody 11–4.1 (anti‐Kk) was reacted with homologous (H‐2k) and heterologous (H‐2b) blastocysts. Positive results with the monoclonal antibody corroborates the concept that H‐2 antigens are expressed on early mouse embryos.