The binding of human Glu- and Lys-plasminogens to intact fibrin clots, to lysine-Sepharose, and to fibrin cleaved by plasmin was quantitatively characterized. On intact fibrin clots, there was one strong binding site for Glu-plasminogen with a dissociation constant, Kd, of 25 microM and one strong binding site for Lys-plasminogen with a Kd of 7.9 microM. In both cases, the number of plasminogen binding sites per fibrin monomer was 1. Also, a much weaker binding site for Glu-plasminogen was observed with a Kd of about 350 microM. Limited digestion of fibrin by plasmin created additional binding sites for plasminogen with Kd values similar to the binding of plasminogen to lysine-Sepharose. This was predictable given the observations that plasminogen binds to lysine-Sepharose and can be eluted with epsilon-aminocaproic acid [Deutsch, D.G., & Mertz, E.T. (1970) Science (Washington, D.C.) 170, 1095-1096] and that plasmin preferentially cleaves fibrin at the carboxy side of lysyl residues [Weinstein, M.J., & Doolittle, R.F. (1972) Biochim. Biophys. Acta 258, 577-590], because the structures of the lysyl moiety in lysine-Sepharose and of epsilon-aminocaproic acid are identical with the structure of a COOH-terminal lysyl residue created by plasmin cleavage of fibrin. The Kd for the binding of Glu-plasminogen to lysine-Sepharose was 43 microM and for fibrin partially cleaved by plasmin 48 microM. The Kd for the binding of Lys-plasminogen to lysine-Sepharose was 30 microM. With fibrin partially cleaved by plasmin, there were two types of binding sites for Lys-plasminogen, one with a Kd of 7.6 microM and the other with a Kd of 44 microM.(ABSTRACT TRUNCATED AT 250 WORDS)