Abstract Thrombin preferentially cleaves fibrinopeptides A (FPA) form fibrinogen resulting in the formation of desAA-fibrin from which most of the fibrinopeptides B (FPB) are tehn released with an enhanced rate. Kinetics of fibrinopeptide release from normal and dysfunctional fibrinogens were investigated in order to further characterize the mechanism of accelerated FPB release during desAA-fibrin polyumerization. Dysfunctional fibrinogens London I and Ashford, exhibiting primary polymerization abnormalities (i.e., an abnormality present when all fibrinopeptides have been cleaved), which in the case of fibrinogen London I is believed to be caused by a defect in the D-domain, were shown to exhibit a decreased rate of FPB release compared with normal fibrinogen. While Gly-Pro-Arg-Pro, an inhibitor of fibrin polumerization, was shown to decrease the rate of FPB release from normal fibrinogen by a factor of 5, normal fragment D 1, although inhibiting clot formation of normal fibrinogen, did not influence the accelaration of FPB release. On the other hand, the presnece of fragment D 1 did not enhance FPB release from fibrinogens London I, suggesting that interaction of D-domains in functional isolation with desAA-fibrin E-domains is not sufficient to enhance FPB release. Although clot formation was inhibited by the concentrations of fragement D 1 used, the formation of small desAA-fibrin oligomers was hardly affected. Thus, small fibrin polymers, but not desAA-fibrin monomers, act as optimal substrates for the release of FPB by thrombin.