Natural killing (NK) capacity was evaluated in peripheral blood mononuclear cells from 14 patients with well defined primary immunodeficiency disorders and compared with the activity of those cells in antibody-dependent cell-mediated cytotoxicity (ADCC) assays against antibody-coated erythrocyte (killed primarily by monocytes) and lymphoid or tumor targets (killed exclusively by lymphoid cells). A selective inability to lyse antibody-coated lymphocyte targets was observed with cells from patients with x-linked agammaglobulinemia, suggesting the involvement of either a different lymphocyte subpopulation or membrane receptor for NK and ADCC, or that a different functional susceptibility exists for the two types of killing. The only immunodeficiency state in which lymphocyte NK activity was found to be lacking was severe combined immunodediciency disease.