Calcium bilirubinate, a water-insoluble salt of bilirubin forming the major component of the pigmented Ca stone, was synthesized in vitro either by addition of calcium hydroxide into solutions of sodium biliru- binate or potassium bilirubinate, or by suspending free bilirubin im- mediately into a solution of calcium hydroxide. No structural change took place in the bilirubin moiety since free bilirubin, associated with the same absorption characteristics as the standard preparation, could be recovered from the synthesized calcium bilirubinate by treatments with HC1 followed by a chloroform extraction.