Abstract Fibrinogen synthesis in rabbits was evaluated following intravenous infusions of stage 3 degradation products of homologous fibrinogen or fibrin, prepared in vitro. Fibrinogen production was measured by determining the rate of appearance of 75SeM into circulating fibrinogen. Fibrinogen synthesis increased threefold after the administration of stage 3 FDP (D and E), dialyzed to remove LMW digestion fragments. In contrast, the fdp obtained by plasminolysis of crosslinked thrombin clots or of noncrosslinked ancrod or thrombin clots failed to enhance basal fibrinogen production. Accelerated fibrinogen production was not accompanied by alterations in haptoglobin concentration or by increased incorporation of 75SeM into haptoglobin. Fibrinogen synthesis was not increased after infusions of FPA and FPB.