Exposure of human neutrophils to low concentrations of phorbol myristate acetate (PMA) results, after a brief lag, in the production of superoxide anion and the phosphorylation of membrane proteins. Evidence that these responses are linked has now been obtained using a monoclonal antibody directed against an undefined macrophage surface antigen. The addition of this antibody, which recognizes a 90 kDa neutrophil membrane protein, caused dose-dependent delays in the onset of both phosphorylation of neutrophil membrane proteins and in the appearance of superoxide anion, following addition of PMA to the cell suspensions. For each response the lag period increased with increasing concentrations of antibody, but the onset of phosphorylation always preceded by a few minutes the initial appearance of superoxide anion.