It is likely that the murine response to human recombinant TNF alpha (hrTNF alpha) may generate antibodies (Ab) to epitopes present in TNF alpha from other species. Here, we demonstrate that F5 anti-hrTNF alpha monoclonal antibody (mAb) recognizes feline TNF alpha while E8 anti-hrTNF alpha mAb failed to do so. In order to demonstrate that E8 and F5 mAb recognize different epitopes in the hrTNF alpha molecule, a constant concentration of E8 and variable concentrations of F5 were incubated with solid phase bound hrTNF alpha. Binding of E8 and F5 to hrTNF alpha was determined with anti-mu and gamma chain specific Ab. F5 bound equally to hrTNF alpha in the presence or absence of E8 and the same amount of E8 bound to hrTNF alpha, in spite of the presence of F5. When using the E8 and F5 mAb for capturing the TNF alpha from the equine, canine, feline and bovine species, in supernatants of an ex vivo lipopolysaccharide (LPS)-stimulated whole blood cell culture, we only detected the feline TNF alpha by F5 mAb (p = 0.001). By a cytotoxic assay on L929 fibroblasts, we indeed demonstrated the feline TNF alpha production after the LPS stimulus. In an inhibition assay, the human and feline cytokines competed for F5, although the inhibition of native human TNF alpha binding to F5 was significant but only about 20% (p = 0.001). In conclusion, most likely the F5 anti-hrTNF alpha mAb recognizes an epitope in feline TNF alpha. Its immunomodulatory potential in the feline model remains to be studied.