Cyclosporin A (CsA) has been reported to inhibit phorbol myristate acetate (PMA)-induced superoxide anion (O2-) formation in human neutrophils and murine macrophages. We found that CsA inhibited O2- formation in HL-60 cells induced by PMA (30 nM) and phorbol dibutyrate (200 nM) with a half-maximal effect at 1 and 0.75 microM, respectively. One possible target of CsA action is protein kinase C (PKC) [EC 2.7.1.37] since phorbol esters activate this kinase. However, CsA did not inhibit PMA-mediated reduction of histamine-induced rises in cytosolic Ca2+ concentration in, and PMA-induced differentiation of, HL-60 cells and platelet aggregation. CsA did not reduce the activity of various recombinant c-PKC isoenzymes (alpha, beta 1 and gamma), n-PKC isoenzymes (delta and epsilon), an a-PKC isoenzyme (zeta) nor of PKC purified from rat brain in vitro. These data show that CsA inhibits phorbol ester-induced O2- formation in HL-60 cells but not other phorbol ester-mediated events and that inhibition by CsA of O2- formation cannot readily be attributed to direct PKC inhibition. We also show that CsA does not change the activity of nucleoside diphosphate kinase [EC 2.7.4.6] in HL-60 membranes nor the latter's physical properties.