Four children with an immunodeficiency involving the absence of leukocyte membrane glycoproteins reacting with anti-LFA-1 and OKM-1 monoclonal antibodies were unable to mediate adherence-dependent leukocyte functions. Even with normal Fc receptor function, their PMN-ADCC and MC-NKC were markedly deficient. Single cell analysis demonstrated deficient antibody-mediated PMN-target cell adherence. Monoclonal antibodies against LFA-1 and OKM-1 reproduced this immunodeficiency in leukocytes from normal adults. LFA-1/OKM-1 mediates a PMN-target cell adhesive step.