Three characteristics of the mechanisms of antibody-dependent macrophage-mediated cytolysis (ADMC) and lectin-dependent macrophage-mediated cytolysis (LDMC) in a C3H/He mouse-MM46 syngeneic tumor system were compared: the role of divalent cations, the effect of cyclic nucleotides, and the role of protease and oxygen compounds as lytic substances. Mg++, but not Ca++, was required for ADMC but neither was required for LDMC. Ca++ inhibited both LDMC and ADMC. Dibutyryl cyclic adenosine 3',5'-monophosphate (cAMP) and its agonists, such as cholera toxin, prostaglandin E1 (PGE1) and E2 (PGE2), inhibited ADMC, but had no effect on LDMC. Dibutyryl cyclic guanosine 3',5'-monophosphate (cGMP) had no effect on either ADMC or LDMC. Isoproterenol and isobutylmethylxanthine had no effect on either ADMC or LDMC. Protease inhibitors, such as pepstatin, bestatin, chymostatin, elastatinal, leupeptin, and bovine pancreatic trypsin inhibitor, did not affect either ADMC or LDMC. Tosylphenylalanyl-chloromethylketone inhibited both ADMC and LDMC, and also inhibited macrophage-tumor cell contact in both systems. Neither catalase nor superoxide dismutase clearly inhibited either ADMC or LDMC. The different sensitivities of ADMC and LDMC to pharmacological agents suggest that these two types of mediator-dependent macrophage-mediated cytolysis have partly different cytolytic processes as well as different recognition sites.