The mechanism(s) of natural killer (NK) cell-mediated cytotoxicity (CMC) remains largely unknown. In this study, we investigated the possibility of human NK cells to exhibit an oxidative burst (OB) after stimulation by K562, an NK-sensitive target cell (TC). The addition of catalase (CAT) or superoxide dismutase (SOD) to the NK-mediated cytotoxic assay had no effect on NK-CMC. In contrast, CAT and SOD effectively modulated the cytotoxicity mediated by phorbol-12-myristate-13-acetate (PMA)-activated polymorphonuclear leukocytes (PMNL) against three different tumor TC, including K562. CAT abrogated, while SOD enhanced PMA-activated PMNL-mediated cytotoxicity. The synergistic effect of SOD and PMA was suppressed in a dose-dependent fashion by CAT. Furthermore, by chemiluminescence (CL) and SOD-inhibitable reduction of cytochrome c, we failed to detect an OB associated with K562-stimulated NK cells. PMNL, however, rapidly responded to PMA (10 ng/ml), generating almost 10(6) cpm within 20 min and 26.7 nM O-2/10(6) cells/30 min, as detected by CL and reduction of cytochrome c, respectively. Finally, K562 alone, at cell concentrations corresponding to effector cell:target cell (EC:TC) ratios of 1:1 and 1:10, reduced cytochrome c, but this reduction was not inhibited by SOD, thus suggesting a diaphorase activity. Overall, we show that: a) tumor cell destruction by human NK cells and by PMA-activated PMNL is mediated by different mechanisms; and b) NK-CMC against a sensitive TC does not involve an OB.