CRYSTALLINE bilirubin changes with time as noted by Kuster1. Newly recrystallized, it dissolves immediately and completely in chloroform. With aged preparations, when dissolved at room temperature, a certain amount of insoluble substance is observed which dissolves within a few seconds when boiled under reflux. This must be due to either an isomerization from bilirubin to dihydrobiliverdin—a displacement of the π electrons from the vinyl to the central methylene bridge2—or a transition between two crystalline phases, the less soluble having the highest thermodynamical stability.