SummaryA study of the removal of I131-labeled Congo red from the blood of amyloid, non-amyloid, multiple myeloma, rheumatoid arthritis and other patients is presented. The percentage removal of the labeled dye shows the same variation reported by many other workers using Bennhold’s test and its modifications.However, there seems to be a positive correlation between the percentage removal of the labeled dye and the presence of amyloid as revealed by biopsy and autopsy. The half-time of disappearance is also correlated with the amyloidosis.The availability of the I131-labeled dye permits the use of very small weights of the dye thereby drastically reducing the possibility of toxic and sometimes fatal reactions encountered with the unlabeled dye. The I131 present permits easy quantitation of the dye in the blood without separation of plasma and obviates the need of fasting. It also permits external counting and scanning of deposits in the organs containing the dye.The availability and use of the labeled dye may stimulate more comparative studies of the removal of the dye from the blood correlated with biopsy and autopsy findings.