Chemical analysis of bilirubin in pigmented calcium stones led to the following conclusions: Bilirubin in such calculi is nearly entirely combined with some inorganic base into almost water-insoluble salt. It is readily freed from the salt by dilute acid, and takes up nearly the entire bulk of extracted bilirubin or 22.3-44.2% of the entire gall-stone specimen, forming the largest component of the stones. About 5.3-23.1% of the stone specimen is made of a bilirubin derivative soluble in NH3 water, consisting at least partly of bisected bilirubin molecules. In normal bile, bilirubin exists in the form of glucuronide or water soluble salt, but in pathological cases, perhaps the combined bilirubin is released in free form and its combination into water-insoluble salt and formation of calcium bilirubinate are facilitated thereby.