In the healthy adult a major source of bile pigment is the breakdown of hemoglobin derived from senescent red blood cells which normally have a lifespan of about 120 days. Two other sources that have been identified are ineffective erythropoesis and catabolism of hepatic hemes. Most of this information has been derived from isotopic labeling experiments. There is growing evidence that, in the absence of a satisfactory conjugating system in the liver, bilirubin can be metabolized by other mechanisms and such mechanisms may operate to a small extent under normal circumstances. In the gut bacterial reduction of bilirubin yields a wide range of pyrrolic compounds but the most highly reduced tetrapyrrole is (-)-stercobilinogen, a colorless lipophilic substance normally present in feces and in trace quantities in urine. The urobilinogens form the most highly reduced class of bile pigments, being colorless but having absorption in the ultraviolet.