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A rat anti-recombinant mouse tumour necrosis factor-alpha (rmTNF-α) monoclonal IgM antibody (1F3F3) with high specific binding activity for rmTNF-α was generated. The 1F3F3 monoclonal antibody (mAb) neutralizes the cytotoxic activity in vitro of rmTN F-α on L929 cells and inhibits the binding of radiolabelled rmTNF-α to its putative receptor on L929 cells. The 1F3F3 mAb binds to monomeric, dimeric and trimeric rmTNF-α and does not bind to reduced rmTNF-α, indicating that the recognized epitope is sensitive to denaturation. Using the 1F3F3 mAb as a capturing antibody and a biotinylated anti-rTNF-α as a detecting antibody, we have developed a sandwich ELISA that can specifically detect biologically active mTNF-α with a detection limit of 10 pg mTNF-α/well. This assay correlates well with the classical L929 cristal violet assay for the detection of bioactive rmTNF- in biological fluids. The 1F3F3 mAb inhibits various in vitro biological activities of the rmTNF-α, such as the TNF-α-mediated tumouricidal activity of activated macrophages, the rmTNF-αdependent stimulation of neutrophil degranulation and the growth-promoting effect of rmTN F-α. In vivo the 1F3F3 mAb inhibits lipopolysaccharide (LPS)-induced endotoxic shock. In conclusion, the 1F3F3 mAb is a useful tool to probe rmTNF-α activity both in vitro and in vivo. [ABSTRACT FROM AUTHOR]