Human neutrophils, activated by phorbol-myristate acetate (PMA), (A- neutrophils), were found to suppress lymphocytic killer (K) cell- mediated antibody-dependent cellular cytotoxicity (ADCC). Resting (R) neutrophils, ie, PMA-untreated cells, were completely ineffective. Suppression was optimal when A-neutrophils were added at the beginning of the ADCC assay. Furthermore, A-neutrophils were found to cause an approximately 80% reduction in the number of Raji target cell-bound lymphocytes. These data indicate that A-neutrophils inhibit K cell activity by interfering with the target cell recognition. A-neutrophils were capable of reducing the percentage of Fc receptor (FcR)-bearing lymphocytes with a half-time of 7.2 minutes, through a process preventable by the serine-protease inhibitors tosyl-lysine-chloromethyl ketone (TLCK) and lima bean trypsin inhibitor (LBTI). Conversely, A- neutrophils caused a very slow decrease in the amount of Raji cell- bound antibodies, as detected by the complement-mediated lytic assay. Thus, only lymphocyte FcR structures seem to be highly susceptible to neutrophil-derived TLCK- and LBTI-inhibitable proteases. Furthermore, supernatants from A-neutrophils were found to inhibit K cell ADCC and lymphocyte binding to Raji target cells. In addition, LBTI prevented the A-neutrophil-dependent and the supernatant-dependent inhibition of both K cell ADCC activity and lymphocyte-target cell conjugate formation. Together these data suggest that A-neutrophils suppress K cell function through a protease-mediated impairment of the FcR binding capacity. The results provide evidence that human neutrophils are endowed with mechanisms to regulate K cell ADCC activity.