We investigated the role of protein kinase C (PK-C) in the activation of cytotoxic peritoneal murine macrophages (M phi) by IFN-gamma or by IFN-beta. Two potent inhibitors of PK-C, 1-(5-isoquinolinesulfonyl)-2-methylpiperazine dihydrochloride (H-7) and retinal, were used. We found that both drugs inhibited in a dose-dependent manner the activation of cytotoxicity induced by IFN-beta, suggesting the requirement for intact PK-C activity in this process. In contrast, neither H-7 nor retinal inhibited the activation of cytotoxic M phi by IFN-gamma, indicating that IFN-gamma acts through a PK-C-independent pathway. The effectiveness of both drugs in inhibiting PK-C in intact M phi was evaluated by measuring the inhibition of induction of c-fos mRNA by L-alpha-1-oleoyl-2-acetoyl-sn-3-glycerol, a process that has been shown to be dependent on PK-C activation. We have found a strict correlation in the dose-dependent inhibition by both drugs of c-fos mRNA induction and activation of M phi by IFN-beta. These results indicate that different pathways of activation are triggered by IFN-gamma and IFN-beta, the former being independent from and the latter dependent on intact PK-C activity.