After vaccinia virus vaccination of human volunteers, local indurations developed within 10 days, and regional adenopathy was detected in half of the individuals. Their peripheral blood lymphocytes (PBL) harvested at different days after vaccination showed specific activity against target cells infected with vaccinia virus with a peak activity at day 7. The specificity of the cytotoxic activity was not related to HLA markers, since autologous, homologous, and heterologous infected target cells were lysed with the same efficiency. The cytotoxic activity was caused by PBL that did not rosette with sheep erythrocytes and could be depleted by more than 90 percent by removing Fc receptor-bearing cells. T-cell- depleted PBL showed a one-half to two times greater cytotoxicity than intact PBL. The cytotoxic activity could also be abrogated by more than 95 percent by rabbit Fab(2) anti-human IgG. On the other hand, nonimmune PBL lysed vaccinia-infected target cells in the presence of specific antibodies against vaccinia virus, thus demonstrating that ADCC could be efficient in lysing vaccinia-infected target cells. We conclude that after vaccination, antibody-forming cells arise and provide specific anti-viral antibody and that the cytotoxic cells detected in this reaction are K cells. These experiments suggest that antibody-dependent cell cytotoxicity may be of major importance in the recovery of man to virus infections.