It was found that fragments E (Mr = 45 000), DH (Mr = 95 000) and DL (Mr = 82 000) decrease the rate of plasmin hydrolysis of fibrin that is not cross-linked with factor XIII; the most effective inhibitor is fragment DL. The Kd values for the interactions of fragments E, DH and DL with plasmin are equal to 0.15, 0.4 and 0.04 microM, respectively.