Pure pigment stones, a minor variety of gallstones characterized by black appearance, were analyzed by infrared absorption spectroscopy. The spectra of these stones resembled those of calcium bilirubinate in position of respective absorption bands, but they were smoother in general aspect than spectra of usual calcium bilirubinate stones. From this and other findings the main constitutents of the black stones have been identified as polymers of bilirubin derivatives. It was also revealed spectroscopically that the stones were associated with carbonate and/or phosphate of calcium at an incidence of 65 percent, and that they were less commonly associated with organic bile components such as cholesterol, bile acids and fatty acids than calcium bilirubinate stones. In order to quantitate "smoothness" of the spectrum, the quotient Q was calculated from intensity readings at three determined positions, including 1624 cm-1 at which the absorption is due to pyrrole rings of bilirubin. The "smoothness" was found to correlate well with "blackness" of the stone, Q being less than 0.30 for 91 percent of pure pigment stones and over 0.30 for all specimens of calcium bilirubinate stone and synthetic calcium bilirubinate.