Immunologic and enzyme digest epitope mapping techniques were used to compare and contrast the interactions of three mouse mAb, designated A10G10, A6, and B6, with human TNF-alpha. These antibodies have previously been shown to protect mouse and human cells from TNF toxicity. ELISA showed that the antibodies recognize predominantly conformational epitopes, and that native TNF binds at least two molecules of the same mAb. Enzyme digestion of native TNF bound to each of the mAb in turn showed that each of the antibodies binds to or sterically masks a large fraction of the exposed surface of TNF. These epitope mapping data were compared with four putative TNF cell receptor binding sites presented in the literature. The results are consistent with the hypothesis that these antibodies prevent TNF-induced cell death by binding to or sterically masking the TNF receptor binding site.