Abstract A sensitive double-antibody radioimmunoassay for the measurement of amyloid P component (AP) in urine is described. The method is linear with AP concentrations in the range 5 to 2500 μg/L, the lower limit of the assay being 5 μg/L intraassay and interassay coefficients of variation ranged from 2.2% to 10.3% and from 3.0% to 9.8%, respectively. The mean urinary excretion of AP in 20 normal subjects was 37 μg/24 hr, range 5 to 102 μg/24 hr. Compared with normal subjects, patients with rheumatic disease had an increased urinary output of AP (mean 98 μg/24 hr, range 5 to 218 μg/24 hr; P < 0.005). In patients with reactive (secondary) amyloidosis the diurnal excretion of AP was markedly increased (mean 615 μg/24 hr), but the urinary AP albumin ratio did not significantly differ from that in patients with nonamyloid kidney disease who were matched with the amyloid group with respect to the degree of proteinuria and impairment of renal glomerular filtration rate.