Surface plasmon resonance and ELISA experiments revealed that recombinant fibrinogen alpha C fragment (residues A alpha 221-610) corresponding to the alpha C domain binds tPA and plasminogen with high affinity. This binding was found to be Lys-dependent and occurred via independent binding sites. Study with truncated variants of the alpha C fragment located these sites in its COOH-terminal half. Binding of tPA and plasminogen to these sites stimulated activation of the latter whereas proteolytic degradation of the alpha C fragment reduced this effect substantially, suggesting the importance of the alpha C domains in regulation of fibrinolysis.