An electron-microscopical study was made on morphological changes in which T-Tbp would undergo during clotting and fibrinolytic process. Morphological appearance of concentrically arranged membrane structure in T-Tbp remained nearly intact during blood coagulation process. T-Tbp, which existed in the sediments following dissolution of fibrin clot by the application of UK, showed an appearance of fine granules adhering to the surface of aggregates of particles through SEM. Through TEM, T-Tbp in the sediments was found to have retained its concentrically arranged membrane structures in most places, while, in some other places the appearance of fused membranes, smaller single vesicles and long sheets of membranes, and the formation of "blebs" etc. were observed. Various morphological changes caused by fibrinolytic substances accompanied the loss in coagulation activities. Our results showed that coagulation activities of T-Tbp must be completely dependent upon the presence of the membrane structures.