Immunization of mice with heavily irradiated syngeneic Moloney lymphoma cells evokes antibodies against the major viral envelope antigen, gp71, and the Moloney virus-induced cell surface antigen (MCSA). A9HT cells, an L-cell subline, react with the antibodies against the viral envelope antigen only; this reaction can be completely inhibited by virus or purified gp71. Reactivity to Moloney lymphoma cells (YAC) was only partially inhibited (maximum 30%) or not at all. This can be attributed to the reaction of the YAC cells with antibodies directed against MCSA, a nonvirion cell surface component according to both biological and biochemical evidence. Antibody-induced capping of gp71 or p15(E) did not change the membrane distribution of MCSA or H-2, indicating that these antigens represent distinct entities on the cell surface. MCSA showed only minimal capping and thereby differed in behavior from both H-2 and virion antigens. gp71 could be capped by the mouse antiserum as revealed by subsequent staining with monospecific anti-gp71 antiserum. Under ordinary test conditions this reactivity is overshadowed by the reaction against MCSA. The lack of MCSA capping reflects a difference in anchorage of this antigen.