Specific rabbit antisera to the major glycoproteins of Friend leukemia virus (gp71) and the mouse mammary tumor virus (gp52) were utilized to study the surfaces of C3H-, DBA-, BALB/c-, and C57BL-transformed and normal cells by immunoelectron microscopy. Antiserum to gp71 showed reactivity with all of the mouse cells tested regardless of strain, virus production, or state of transformation. In cells producing murine leukemia virus, budding viruses and other areas of the cell surface were consistently labeled with gp71 antiserum. A gp52-like antigen was likewise detected on both cell surfaces and virions of C3H and DBA cells producing the mammary tumor virus. Budding virions with surface spikes but with crescent-shaped nucleoids in C3H/HeJ cells were labeled specifically with gp52 antiserum. The antigen localized with anti-gp52 serum was detected in low concentration on the surface of nonvirus-producing cultures of a C57BL/6 sarcoma induced by the Schmidt-Ruppin D strain of Rous avian sarcoma virus (SRD-2), a BALB/c bone marrow culture (JLS-V9), and a normal BALB/c fibroblast culture (BALB/cF). Other cell cultures transformed by either C-type virus or methylcholanthrene failed to demonstrate gp52 antigen. Both gp52- and gp71-like antigens were found to be expressed simultaneously in C3H/HeJ, C3H-MT, DBA-MT, SRD-2 (transformed) and BALB/cF, JLS-V9, and C3H-1 (normal) cultures. Expression of gp52 antigen in the absence of gp71 was not detected in any of the cultures examined. These findings demonstrate the ubiquitous expression of gp71 in a wide variety of normal and transformed mouse cells while gp52 tends to be expressed predominantly in cells from mice with high mammary tumor incidence (C3H) and DBA), but only to a minor extent in cells from low mammary tumor incidence strains (BALB/c and C57BL).