The magnitude of the cytotoxic T lymphocyte response in both immunologically naive mice and in mice primed with a serologically non-cross-reactive influenza A virus was greatly diminished if the virus used for challenge was first incubated with excess hemagglutinin-specific monoclonal antibody. However, the level of T cell stimulation was, when compared with virus stocks that are simply diluted, greater than would be expected from the infectivity titer remaining after exposure to the monoclonal antibody. Also, mice given mixtures of virus and monoclonal antibody were primed for a secondary cytotoxic T lymphocyte response, the frequency of memory T cells being approximately fourfold lower than that found for animals injected with virus alone. The possibility that such virus-antibody mixtures might be of value for immunization is worth further investigation.