Phorbol esters rapidly induce aggregation of human mononuclear leukocytes in vitro. Previous studies have indicated that cell surface proteins are involved. We report now that the monoclonal antibody 60.3, either as purified IgG or as Fab' fragments, to an antigen common to leukocytes completely inhibited the phorbol ester-induced intercellular adhesion (binding). No inhibition of cell aggregation was observed with monoclonal antibodies to common leukocyte antigen T 200, T-cell-associated antigen, monocyte-granulocyte antigen, brain granulocyte-T-lymphocyte antigen, transferrin receptor, mature T-cell antigens (mol.wt either 67,000 or 19,000/29,000), T helper/inducer cell antigen, sheep erythrocyte receptor, class I or class II antigens, or T cytotoxic/suppressor cell antigen. The antibody 60.3 did not inhibit stimulation of the cells since the characteristic phorbol ester-induced morphological changes and phorbol ester-enhanced cap formation of membrane glycoproteins were readily observed. Two major cell surface polypeptides with apparent molecular weights of 90,000 and 160,000 were immunoprecipitated. We conclude that this protein complex, or at least one of its components, mediates adhesion among mononuclear leukocytes.