Antisera raised in rabbits against either purified recombinant-derived human tumor necrosis factor (TNF)- alpha (huTNF) or huTNF peptide-bovine thyroglobulin conjugates were evaluated for anti-equine TNF (eqTNF) activity. Binding and neutralizing anti-eqTNF activities were found in antisera raised against whole huTNF or against either of the peptides containing the N-terminal 15 amino acids of huTNF (huTNF[1-15] and huTNF[1-31]). Anti-eqTNF activity was not detected in antisera raised against huTNF[65-79], huTNF[98-111] or huTNF[124-141] peptides. The addition of excess huTNF[1-15] completely inhibited the ability of anti-huTNF[1-15] to bind eqTNF and reduced by approximately 25% the anti-eqTNF activity of an antiserum raised against whole huTNF. Nonconjugated huTNF[1-15] did not have eqTNF agonist or antagonist activity. Results were consistent with previous structural and functional data implicating the N-terminus of huTNF in receptor binding and indicate that the homologue of huTNF[1-15] on eqTNF may be a potentially important target for neutralizing anti-eqTNF antibodies.