A series of strategically designed recombinant (r) mutants of the kringle 1 region of human plasminogen ([K1HPg]) have been constructed and the resulting gene products employed to reveal the identities of the residues that contribute to stabilization of the binding of omega-amino acid ligands to this domain. On the basis of determinations of the binding constants of the ligands, 6-aminohexanoic acid and trans-4-(aminomethyl)cyclohexane-1-carboxylic acid, to a variety of these mutants, we find that the anionic site of the polypeptide responsible for stabilization of the amino group of the ligands consists of both D54 and D56 and the cationic site of the polypeptide that interacts with the carboxylate group of the ligand is composed solely of R70. The main hydrophobic interactions that stabilize binding of these ligands, likely by interactions with the ligand hydrophobic regions, are principally due to W61, Y63, and Y71. The results obtained are consistent with conclusions that could be made from analysis of the X-ray crystal structure of r-[K1HPg] and from previous studies from this laboratory regarding the binding of ligands of this type to the kringle 2 region of tissue-type plasminogen activator ([K2tPA]). It thus appears as though a common ligand binding site has evolved in different kringles with ligand specificity differences between r-[K2tPA] and r-[K1HPg] perhaps explainable by the different nature of the cationic sites on these polypeptides that are involved in coordination to the ligand carboxylate groups.