The three major immunocompetent cells in human peripheral blood (lymphocytes, neutrophils, and monocytes) were shown to be effector cells for antibody-dependent cell-mediated cytotoxicity (ADCC) against influenza virus-infected baby hamster kidney cells in vitro. Lymphocyte cytotoxicity was mediated by FcIgG receptor-bearing null cells and Ty cells. These effector populations were best defined by HNK-1, a monoclonal antibody to human natural killer and ADCC-mediator cells. Antibody responsible for ADCC against influenza virus-infected cells was detectable in sera of young children after natural infection and after vaccination with inactivated and live attenuated viruses. ADCC antibody appeared before hemagglutination-inhibiting antibody and persisted for at least one year after vaccination with live attenuated vaccine. ADCC antibody was subtype-specific but quite broadly reactive within a subtype. Both hemagglutinin and neuraminidase were antigenic determinants for ADCC antibody. An anamnestic response to the original strain was observed after challenge with influenza virus of a heterologous subtype.