Conventional antisera and monoclonal antibodies have been used to demonstrate the presence of H‐2 antigens on preimplantation mouse embryos. In this study, the specificity of previously used anti‐H‐2 conventional antisera was tested by absorption of the antisera on spleen lymphocytes. It was found that absorption of the anti‐H‐2 antisera removed all of the antibody reactive with lymphocytes, but only about half of the antibody reactive with embryos. The effect of absorbed and nonabsorbed anti‐H‐2 antisera on mouse blastocysts was tested both by cytotoxicity assays and by electron microscopy. The results suggest that conventional antisera do indeed detect H‐2 antigens on mouse embryos, but they may detect other, as yet undefined, antigens as well.