A chromium release assay was established to study human antibody-dependent cell-mediated cytotoxicity (ADCC) of HEp 2 cells infected with respiratory syncytial (RS) virus. Human peripheral blood lymphocytes in the presence of specific antibody to RS virus caused in vitro lysis of RS virus infected target cells. ADCC was detected in sera of infants with RS virus infections and in specimens of colostrum. The ability of serum or colostrum to mediate the cytotoxic reaction appeared to be related to the level of specific IgG, or IgA antibody to RS virus, as detected by membrane fluorescence. Separation of effector cells by their glass adherence properties showed that the ability to produce cytotoxicity resided in non-adherent effector cells.