Simultaneous studies of the metabolism of 125I-prothrombin and 131I-fibrinogen were performed in ten patients with thrombocytosis secondary to myeloproliferative syndromes. The mean fractional catabolic rate (FCR) of prothrombin was 61.9%, differing significantly from the normal mean of 42.5% (p <0.01). Similarly, the mean FCR of fibrinogen was 34.0%, differing significantly from the normal mean of 23.9% (p <0.01). Excellent correlation existed between the increased FCRs of prothrombin and fibrinogen but not between these FCRs and the platelet level, at least for platelet counts above 1,000,000/cu mm. Therapeutic reduction of platelet counts to the normal range was associated with a normalization of FCRs as well.