To determine the metabolic fate of factor X in primary amyloidosis associated with factor X deficiency, we examined the pathways of its catabolism in a man with this syndrome. Intravenous infusion of human or bovine 131I-labeled factor X established a triphasic plasma clearance pattern for factor X. About 85 per cent of the factor X disappeared, with a disappearance half-time of less than 30 seconds. A second and third phase showed a T1/2 of 90 minutes and nine hours respectively. 131I-labeles factor X in plasma did not appear to be rapidly modified or degraded. Relatively minor quantities of 131I were cleared into the urine. We observed a diffuse distribution of radioactivity over the body surface, with a concentration in the hepatic and splenic regions. These studies demonstrate than factor X deficiency associated with systemic amyloidosis is due to binding of factor X to body tissue, probably within the circulatory system.