Uricine was found to be present in all uric acid stones tested, although another physicochemically related pigment was occasionally present. Uricine is of low molecular weight (282), and a preliminary analysis of its structure suggests that it is formed by two pyrrolic rings. It is a yellow pigment whose color is pH dependent, shifting from reddish yellow to compounds at pH 1.0. It is water soluble, but its solubility increases in alkaline solutions. Preliminary evidence suggests that uricine is related metabolically to bilirubin like compounds.