The receptor-recognition site human transforming growth factor-alpha (TGF alpha), a 50-residue tricyclic peptide with three disulfide bonds, was mapped by a set of 46 peptide analogs consisting of linear, monocyclic, bicyclic, and tricyclic structures representing individual and overlapping subdomains of human TGF alpha. Linear overlapping fragments ranging from 7 to 18 residues and spanning the entire length of TGF alpha as well as monocyclic analogs with one disulfide linkage were found to be inactive in both receptor-binding and mitogenic assays. Bicyclic analogs with two disulfide linkage and representing either the amino or carboxyl two-thirds of TGF alpha showed low activity at 0.1-0.9 mM concentrations. Tricyclic analogs containing all three disulfide linkages but lacking either the amino or carboxyl terminal heptapeptide was, respectively, 3% and 0.1% as active as TGF alpha. These results show that determinants for the receptor binding cannot be represented by a short continuous fragment or a single subdomain, but are located on a discontinuous surface on a folded structure with disulfide restraints. Furthermore, these results when combined with our previous results which shows that the middle subdomain (second disulfide loop) is not involved in the receptor binding suggest that the receptor-binding residues are constituted of three fragments located at the first and third subdomains as well as the external carboxyl peptide.