Human neutrophils incubated with the anti-HLA-DR mAb Lym-1, plus PMA, induced significant cytolysis of B lymphoma cells compared with Lym-1 and PMA alone. The effect of PMA was independent of the ability of the compound to stimulate neutrophil-respiratory burst. In fact, first, neutrophils from a patient with chronic granulomatous disease were cytolytically effective in spite of their inability to produce oxidants. Second, various kinase inhibitors exerted different effects on the PMA-stimulated cytolytic system and neutrophil-oxidative burst. Previous studies have shown the involvement of the FcgammaRII, CD11b-CD18 integrins, and CD66b glycoproteins in the Lym-1 mAb-dependent cytolysis by GM-CSF-stimulated neutrophils. The present PMA-stimulated system was inhibited by the anti-FcgammaRII mAb IV.3, the anti-CD18 mAb MEM 48, and the anti-CD11b mAb 2LPM19c but not by the anti-CD66b mAb 80H3 and N-acetyl-D-glucosamine. Furthermore, the PMA- and GM-CSF-stimulated cytolysis was insensitive and sensitive to inhibition by pertussis toxin, respectively. Thus, the use of PMA and GMCSF as neutrophil stimulants uncovers the existence of distinct mechanisms of Lym-1 mAb-mediated cytolysis.