In previous studies we showed that tumor-associated macrophages isolated from murine mammary tumors are mutagenic to bacteria and mammalian cells and thus may contribute to tumor progression. We reported previously, and confirm here, that inflammatory macrophages induce DNA strand breaks in cultured mammary tumor cells co-incubated at a 1:1 ratio for 1 h. This activity is prevented by inhibitors of arachidonate metabolism or the removal of H2O2 with catalase. In the present study, we show that two antibodies to recombinant murine tumor necrosis factor alpha (rMuTNFa)--a hamster monoclonal antibody (TN3-19.12) and a rabbit polyclonal antibody (Genzyme)--partially protect tumor cells from DNA strand breaks induced by elicited but not resident peritoneal macrophages. Antibody protection was reversed upon the addition of excess exogenous rMuTNFa. Purified rMuTNFa alone was unable to induce DNA strand breaks in the absence of macrophages, indicating that TNFa is necessary but not sufficient to mediate damage. Tumor target cells were completely resistant to the cytotoxic effects of rMuTNFa in the absence of actinomycin D and relatively resistant (in comparison to WEHI 164 clone 13 cells) in its presence. The incomplete protection seen with either catalase or anti-TNF suggests that macrophage-released TNFa, in the presence of other factors, induces non-cytotoxic DNA effects in tumor cells.