Human peripheral blood lymphocytes (PBL) harvested after vaccination with vaccinia or measles virus showed a specific activity against virus-infected target cells. This activity peaked on day 7 and was specific for the target cells infected with the virus used for the vaccination. The cytotoxic activity was not related to HLA markers. The cells involved in the cytolytic process were lymphocytes bearing Fc receptors. In addition, the cytotoxic activity was abrogated by more than 90% by rabbit Fab'2 anti-human IgG. It is therefore likely that two subpopulations of lymphocytes are involved: an antibody-secreting cell providing specific antiviral antibody and an effector cell bearing Fc receptor (K cells). Finally, these experiments suggest that antibody-dependent cell cytotoxicity may play a major role in the recovery from virus infection in man.