Stimulated platelets secrete a variety of physiologically active substances that affect many neutrophil functions. We have examined the capacity of platelets to modulate superoxide anion generation by neutrophils. The amounts of superoxide anion produced by neutrophils in the presence of platelets were markedly enhanced when platelet-neutrophil coincubations were stimulated with agents that simultaneously activate both cell types, as the calcium ionophore A23187 and sodium arachidonate. This effect was dependent upon the number of platelets added to the incubation media and was not affected by inhibitors of arachidonic acid pathway or by preincubation of platelets with an antibody anti-P-selectin. The hypothesis of an involvement of purine nucleotides released by platelets during aggregation on the observed effect of enhancement of superoxide anion generation by neutrophils was then tested. Experimental evidence indicates that platelets release, during A23187-induced aggregation, amounts of ATP that are of the same order (5-10 microM) of those demonstrated to enhance superoxide anion generation by neutrophils. In addition, platelet lysates mimicked the effect of intact platelets in enhancing superoxide anion generation by A23187 stimulated neutrophils. Interestingly, at variance with the results obtained with intact platelets and platelet lysates, supernatants of thrombin-stimulated platelets did not increase O2.- by neutrophils. The enhancing effect of these supernatants was, however, restored when platelets were preincubated with an antibody anti P-selectin. These data indicate that platelets, through the release of purine nucleotides, enhance superoxide generation by neutrophils, thus increasing the cytotoxic potential of these cells.