Unlike resident peritoneal macrophages (RPM) or tumor necrosis factor a (TNFa)primed bone marrow-derived macrophages (BMM), unprimed BMM do not generate superoxide in response to the protein kinase C (PKC) activator, phorbol myristate acetate (PMA). However, these cells do contain significant levels of PKC activity. In contrast to PMA, zymosan induces the generation of superoxide in unprimed BMM, as well as in TNFa-primed BMM and RPM. Staurosporine, a potent PKC inhibitor, failed to affect the zymosan-induced production of superoxide by unprimed and TNFa-primed BMM and RPM, in spite of substantial inhibition of PMA-induced superoxide production by the primed BMM and RPM. However, when PKC was depleted from unprimed BMM by prolonged (24 h) treatment with phorbol dibutyrate (PdBt) (lo-' M) the ability of rymosan to induce the production of superoxide was greatly diminished. Such a result could be interpreted as suggesting a role for PKC in the zymosan-induced response, a conclusion which contrasts with the inhibitor data. However, PKC depletion, in this case, is achieved via the PdBt-induced activation of PKC. It is thus possible that it is the initial activation of PKC, rather than its depletion, that suppresses superoxide production. Consistent with this interpretation, the co-stimulation of unprimed BMM with both zymosan and PMA resulted in a reduced superoxide release compared to zymosan alone. The activation of PKC therefore appears to have a suppressive effect on the generation of superoxide by unprimed cclls. We thus conclude that PKC is not required for zymosan-induced supcroxide production by either primed or unprimed macrophages and suggest that PKC may be involved in regulatory mechanisms restricting superoxide production by macrophages. However, since PMA alonc can initiate the release of superoxide from primed BMM and RPM, it would appear that PKC can mediate both stimulatory and suppressive signals for macrophage superoxide production.