125I-Serum amyloid P component (SAP), injected intravenously into 10 normal subjects, remained predominantly intravascular with mean (SD) T1/2 (half time) in plasma of 24.5 (5.9) h. The fractional catabolic rate of 68 (19)% of the plasma pool per day was more rapid than other reported human plasma proteins. All radioactivity was excreted in the urine by 14 d. In 16 patients with monoclonal gammopathy or chronic inflammatory diseases, but without amyloidosis, 125I-SAP metabolism was normal. However, among 45 patients with biopsy-proven systemic amyloidosis (25, amyloid A type; 20, amyloid L type), 125I-SAP was cleared from the plasma more rapidly, accumulated in the amyloid deposits, and persisted there. The T1/2 in amyloid, measured directly with 131I-SAP, was 24 d. Repeat studies after 6-18 mo were notably consistent in normals but changed significantly in amyloid patients, generally correlating with clinical signs of disease progression. Measurements of 125I-SAP turnover may thus be of value for diagnosis and monitoring of amyloidosis. Analysis of SAP metabolism in amyloidosis suggests that plasma SAP is in dynamic equilibrium with a very large amyloid pool, and in two autopsies the total mass of SAP in the amyloid deposits was 2,100 and 21,000 mg, respectively.