Generic term denoting intermediates in the conversion of bilirubin to stercobilin by reductive enzymes in intestinal bacteria. Included are mesobilirubin, mesobilane, mesobilene-b, urobilinogen, urobilin, reduction products of mesobilane (stercobilinogen) and mesobilene (stercobilin), and mesobiliviolin; most are found in normal urine and feces. Products related to these intermediates and found in pathologic conditions (e.g., jaundice, liver disease) are the structurally indefinite probilifuscins and propentdyopents found in gallstones.