AbstractMouse oocytes at the dictyate and metaphase II stages as well as fertilized eggs have been studied by indirect immunofluorescence for the expression of H‐2 histocompatibility antigens on surface membranes. Serologically specific reactivity to H‐2 antibody was observed as patchy fluorescence distributed over the surface of the oocyte membrane. In contrast, one‐cell zygotes exhibited variable reactivity, and early two‐cell stages were negative. Absorption studies confirmed the serologic specificity of the reactivity on oocytes, which could be shown to be due to H‐2 antibody. The results suggest that fertilization results in altered expression of major histocompatibility complex surface antigens, and confirms earlier studies that cleavage stage mouse embryos are not reactive with H