Inhibitors of fibrin stabilization of apparently autoimmune origin, found in two severely bleeding unrelated patients (W. G. and G. A.), were compared with regard to their biological target specificities, potencies and immunological characteristics. Both interfered only with the activation of fibrin stabilizing factor (coagulation Factor XIII) and, while totally preventing the conversion of this zymogen to the functional transamidating enzyme, fibrinoligase (Factor XIII(a)), they showed very little inhibition toward the enzyme itself. Thus, according to the classification of Lorand concerning biological specificities, both can be characterized as Type I inhibitors of fibrin stabilization. Potencies of the two inhibitors were quite similar when measured in conjunction with the plasma zymogen, but they differed remarkably in tests with platelet Factor 13. The inhibitor of patient W. G. prevented the activation of the zymogen from platelets, but that of G. A. had no effect on the platelet factor. It may therefore be concluded that the inhibitor of W. G. is directed exclusively against the a subunit which is a common constituent of plasma as well as platelet factors. The inhibitor of G. A., however, must be targeted against determinants uniquely characteristic for the ab ensemble of the plasma zymogen including the b subunit. On the basis of this difference in target specificity, the inhibitor of W. G. is designated as Type I-1 and that of G. A. as Type I-2. The inhibitors of both patients were isolated as immunoglobulins, and neutralization tests revealed that the antibody of W. G. comprised mainly heavy chains of the IgG1 and light chains of the kappa class. The antibody of G. A. proved to be considerably more heterogeneous and contained IgG1 and IgG3 heavy chains as well as kappa- and lambda-light chains. The finding that the antibody of W. G. inhibited conversion of platelet Factor 13 and also its thrombinmodified form, but had no effect on the thrombin and Ca(2+)-activated factor, is an indication that antigenic determinants existing both on the native zymogen and on its hydrolytically modified form become buried in the Ca(2+)-dependent step of activation. This is clear evidence for the occurrence of a significant conformational change in the protein structure attendant to the process of unmasking of its enzymic activity.