(C57BL/6♀ × C3H/Anf ♂) F1 (B6C3F1) mice were evaluated for immunologic reaction to endogenous murine leukemia virus (MuLV). By immunoelectron microscopy (IEM) it was demonstrated that throughout life these mice possess significant levels of free serum antibody specific for MuLV envelope antigen(s). Immunoglobulin and endogenous MuLV antigen(s), presumably in the form of immune complexes, became localized within kidney glomeruli with age; this deposition correlated well with the chronic development of glomerulonephritis. Immunoglobulin eluted from kidneys of aged animals also was shown by IEM to be specific for virus envelope antigen(s). Although the function of antibody in B6C3F1 mice cannot be positively defined from these studies, it may be 1 critical component in the systemic regulation of levels of infectious endogenous leukemia virus.