Synthesis of H-2 antigens by preimplantation mouse embryos is first detectable at the late blastocyst stage. These antigens were detected using immune precipitation assays of extracts of embryos labeled by incorporation of radioactive amino acids but not by surface iodination. Experiments using isolated inner cell massess and trophoblast vesicles indicate that it is the cells of the inner cell mass that synthesize these antigens. H-2 antigens were not detected in either early blastocysts or at earlier cleavage stages.

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