Abstract Kringle 4 of human plasminogen has been studied by NMR spectroscopy to define the solution structure of the kringle-fold and to characterize the ω-aminocarboxylic acid binding site. Aromatic and aliphatic resonances of the NMR spectrum have been identified with the aid of spin-decoupling and NOE procedures as well as pH-titration and metal ion probe studies. Comparison of the NMR spectrum of kringle 4 with the spectra of various kringle 4 species chemically modified at defined positions permitted the assignment of several resonances to specific residues in the kringle 4 sequence. The NOE studies revealed that Leu 45 is in close proximity of the sequentially distant Trp 25/Trp 61 residue pair, thus delineating a definite structural feature of the kringle-fold. The binding of 6-aminohexanoic acid to kringle 4 was shown to cause shifts in the resonances of several aromatic residues, including those of Trp 71, suggesting that several aromatic residues may be lining the ω-aminocarboxylic acid binding site. The binding of the ligand is competitive with the binding of lanthanide ions which reveal much detail of this site.