After partial digestion of human plasminogen with elastase, followed by chymotryptic cleavage of one of the fragments produced, two polypeptides with molecular weights of approximately 10000 and with lysine‐binding sites still intact were isolated by means of affinity chromatography and gel filtration.

One fragment, which was completely sequenced (88 residues), was identified as the fourth kringle, whereas the other, according to partial sequence analysis represented the first kringle. Equilibrium, dialysis against 6‐aminohexanoic acid yielded for the first kringle one high‐affinity binding site (K_a= 60 mM⁻¹) and for the fourth kringle one single low‐affinity binding site (K_a= 28 mM⁻¹). Moreover, interactions were detected between the first kringle and the N‐terminal CNBr fragment of plasminogen and also fibrin. In these cases an additional lysine‐ binding site, though of low affinity, appears to be involved. Thus, the first kringle seems to play important roles, structurally by contributing to the maintenance of a compact structure of plasminogen through an intramolecular interaction with its N‐terminal polypeptide region, and functionally by increasing the fibrin affinity of Lys‐plasminogen (plasminogen lacking the first 76 residues) and plasmin.