THE bacteria of the large bowel reduce bile bilirubin to colourless, Ehrlich-positive chromogens, collectively termed urobilinogen1. It has been stated that urobilinogen is partly re-absorbed from the colon, and afterwards re-excreted in the bile, and, to a lesser degree, in the urine2. Earlier attempts to provide direct proof for this concept were marred by the limitations of conventional methods for identifying and estimating absorbed chromogen. In order to surmount these difficulties, a labelled urobilinogen, mesobilirubinogen-14C, was prepared from bilirubin-14C (ref. 3), and preliminary investigations of its intestinal absorption and re-excretion were performed in rats.