The expression of Moloney leukemia virus on the surface of a viral-induced lymphoma cell, availability of the virus to anti-viral antibody, and the nature and extent of activation of the complement system during the cell cycle were studied in vitro. Viral antigen was present on the cell surface, accessible to antibody, and was able to activate complement in the presence of antibody throughout all cellular growth phases, while cytotoxicity was confined to the G(1) phase of cell growth. In addition, when cells were arrested in metaphase, viral antigen could be demonstrated on the cell surface by immunofluorescence, and budding virus was seen by electron microscopy. All nine components of complement were activated on the addition of antibody throughout the cell cycle. Additional experiments indicated that in the presence of antibody, C3 and/or C4 were immunospecifically bound to viral-induced lymphoma cells throughout the cell cycle as a result of complement activation. These results indicate that the inability to lyse the cells in the presence of specific anti-viral antibody and complement during the logarithmic phase of cell growth is not due to the lack of expression of Moloney virus antigen(s) on the cell surface, inaccessibility of this surface antigen(s) to antibody, or failure to activate the complement effector system.