The effects of L-lysine, L-arginine, and lysine-arginine association on human neutrophil function were studied. The combination of the two agents, used at pharmacological concentrations, was able to increase both the phagocytosis-associated and the f-MLP-induced chemiluminescence, in the presence of luminol as amplifier agent. Neither superoxide anion production nor phytohaemagglutinin-induced aggregation were affected by the lysine-arginine association. The two amino-acids did not show any effect when used as single agents. L-canavanine (competitive inhibitor of nitric oxide synthesis starting from L-arginine) did not reduce chemiluminescence. These results show that the association lysine-arginine may increase the microbicidal potential of human neutrophils, probably by increasing the production of hypochlorous acid. In addition, a role for nitric oxide in these effects can be excluded.