We characterized the interaction between murine TNF (mTNF) and the neutralizing monoclonal antibodies TN3 and 1F3F3. The epitopes were localized by comparing the detection efficiency for a panel of TNF chimaeric proteins and site-specific muteins in ELISA. Mutation of mTNF amino acid Q131 inhibited the interaction with 1F3F3, whereas mutation of D71/Y72 inhibited the binding to TN3. As D71/Y72 are located in an exposed loop near the TNF intersubunit groove, binding of TN3 promoted the dissociation and/or interfered with the reassociation of subunits into mTNF trimers. 1F3F3, on the other hand, prevented the spontaneous dissociation of bound (hetero)trimeric mTNF.