Local and generalized changes in coagulation may be important in the genesis of vegetations and embolism in infective endocarditis. To characterize such alterations, serial hematological investigations were performed on nine consecutive patients who satisfied the inclusion criteria. Platelet survival was measured by Indium111 labeling. Acute and convalescent samples were analyzed for fibrinogen, factor VIIIc, antithrombin III (AT III), fibrin/fibrinogen degradation products (FDPs), and platelet aggregation. The results suggest that in the active stage of the disease: (1) hypercoagulability may be caused by a rise in acute phase reactants, (2) an acceleration of coagulation and fibrinolysis may supervene, and (3) in some cases there is a reduction in platelet aggregation, possibly as a result of continued circulation of previously activated "exhausted" platelets.