The effect of phorbol esters and mezerein pretreatment on macrophage (M phi) activation for tumor cytolysis, tumor necrosis factor (TNF) secretion, and TNF-alpha mRNA expression was investigated. Following pretreatment with various concentrations (0.01 to 10 micrograms/ml) of phorbol 12-myristate 13-acetate (PMA), phorbol-12,13-dibutyrate (PDBu), or mezerein for 16 h, murine peritoneal M phi were activated with M phi-activating factor (MAF) or calcium ionophore A23187 and tested for cytotoxicity in a 24-h cytolysis assay against 125-I-UdR-labeled P815 mastocytoma and NS-1 myeloma target cells. It was found that pretreatment with all three protein kinase C (PKc) activators inhibited M phi activation for cytotoxicity against P815 cells in a dose-dependent manner. Fifty percent inhibition was achieved at concentrations less than 0.1 micrograms/ml. The inhibition was partially reversible. In contrast, the pretreatment did not at all inhibit but significantly enhanced M phi activation for cytolysis against NS-1 cells. Furthermore, exposure to PMA augmented M phi activation by MAF and A23187 for TNF secretion upon stimulation with trace amounts of lipopolysaccharide (LPS). Although the pretreatment neither enhanced nor significantly reduced the synergistic effect of MAF and A23187 on TNF-alpha mRNA expression, it did increase the expression stimulated by LPS alone. Finally, the PKc activity in M phi treated with PMA, PDBu, and mezerein was down-regulated to about 10% of control. Taken together, our results suggest that: 1) PKc plays an important role in the transduction of activating signals for M phi activation by MAF and A23187 to mediate cytotoxicity against some (P815) but not other (NS-1) tumor cells, 2) the induction of TNF-alpha mRNA expression and TNF secretion may be achieved via a PKc-independent pathway, and 3) M phi are equipped with more than one signal transduction pathways for affecting distinct functional activities.