The noninvasive detection of amyloid deposits has recently become possible using scintigraphy with radiolabelled amyloid precursor protein (131I-beta 2-M). We studied 131I-beta 2-M scintigraphy in 15 dialysis patients with special attention to the mechanism of the localized accumulation of tracer. Scintiscans were positive in 8 of the 15 patients. Those with histologically proved amyloid deposits had positive images at the large joints with a continuous accumulation of tracer after 48, 72 and 144 hours. The most significant accumulation occurred bilaterally in the hip joints, followed by the shoulders and knees. Mild uptake was observed in the hands, elbows and feet. No apparent uptake of tracer occurred in patients with secondary hyperparathyroidism, pseudo-gout or AA-type amyloidosis. Three patients were operated for CTS during this study. The radioactivity of tissue excised during the operation was 2-18 times higher than that of fat or plasma. Study of synovial tissue showed that some of the cells surrounding the amyloid deposits took up the radiolabelled tracer. The present study suggests that circulating 131I-beta 2-M is taken up by the cells surrounding the amyloid deposits and detected as a positive scan. It is not known whether these cells react secondarily to amyloid deposits, or whether they take up beta 2-M and form amyloid fibrils.