Possible interaction of alpha-2-antiplasmin with fibrinogen, fibrin and their fragments independent of factor XIII as well as the inhibitor effect on the Glu-plasminogen activation by tissue activator were studied. It was shown that alpha-2-antiplasmin is adsorbed on desAA- and desAABBfibrin films (Kd 69.0 +/- 1.0 nM 68.6 +/- 5.3 nM, respectively). Glu-Plasminogen has no effect on the inhibitor binding with desAABBfibrin. Alpha-2-antiplasmin shows strong affinity for fibrin D-dimer (Kd 65.0 +/- 4.0 nM) and D-fragment of fibrinogen (Kd 119.0 +/- 21.0 nM), but it does not interact with E-fragment. The inhibitor inside the fibrin clot decreases 10 times the activation rate of Glu-plasminogen by the tissue activator both is the presence and without factor XIII at physiological ratio of Glu-plasminogen, tissue activator, fibrin and alpha-2-antiplasmin. Thus we have shown that fibrinogen/fibrin binds alpha-2-antiplasmin independent of the factor XIII. Binding sites of the inhibitor are localized in D-fragment of fibrinogen and/or fibrin D-dimer. Alpha-2-antiplasmin inhibits the Glu-plasminogen activation by tissue activator on fibrin.