Mouse monoclonal antibodies directed against different epitopes of the murine leukemia virus envelope protein gp70 were used to study the expression of retroviral related env proteins during mouse oocyte maturation, fertilization and blastocyst formation. A gp70-related antigen was detected with immunohistochemistry in growing oocytes but not in primordial and primary non-growing oocytes. Atretic oocytes were also negative. Both parthenogenetically activated and fertilized oocytes were positive. Two-cell stages showed a patchy distribution of the antigen. Later preimplantation stages were negative except for a markedly positive period during compaction of the morula and adhesion of the blastocyst.