Investigation of the properties of indigo carmine reveals that it may be more suitable than the dye, T-1824, for circulatory studies that are now commonly carried out using the dye dilution principle of Stewart and Hamilton. Comparison is made of simultaneous dilution curves of indigo carmine and radioactive iodinated serum albumin. Apparently, indigo carmine is bound to protein firmly enough to remain the plasma in one passage through the heart and lungs, but not too firmly to be extracted rapidly from the plasma by excretory systems in the kidney and liver. This allows repeated injections without accumulation.