Cord blood lymphocytes, monocytes, and neutrophils from newborns were shown to mediate antibody-dependent cellular cytotoxicity (ADCC) against influenza virus-infected cells. Antibody mediating ADCC was detectable in cord plasma, indicating that all components necessary for ADCC against influenza virus-infected cells are present in newborns. Among adult lymphocytes, two effector cell populations of influenza ADCC are recognized: non-T and T gamma cells. Each of these cell types expresses an antigen recognized by monoclonal HNK-1 antibody. The proportion of HNK-1 antigen-positive lymphocytes in cord blood was markedly lower than in adult blood; furthermore, ADCC was mediated by cord blood lymphocytes which were HNK-1 negative. By lymphocyte fractionation, the effector lymphocytes in cord blood were, as in adults, non-T and T gamma cells, suggesting that HNK-1 antigen is not expressed on these cell lineages in newborns.