Conjugates of bilirubin were studied in normal bile of man and rat, and in bile of liver patients. In general human bile was obtained by duodenal intubation. In addition T-tube bile was examined in patients operated on for mechanical obstruction. The bile pigment compositions of duodenal and T-tube bile were similar in two patients where comparison was possible. Obstruction of the bile duct in rats was used as an animal model for obstructive jaundice. Diazotized ethyl anthranilate was used for determination of total conjugated bile pigment and for thin-layer chromatography (t.l.c.) analysis of the derived azopigments. The available t.l.c. procedures are versatile and allow rapid and quantitative analysis. A variety of conjugated azopigments can be distinguished. With chloroform, negligible amounts of unconjugated bilirubin are extracted from bile of man. Therefore, the percentage of monoconjugated bile pigments present in the initial bile sample can be calculated from the percentage of azodipyrrole found after diazotization. Normal bile from man and rat yields similar azopigment patterns. The dominant component is azopigment-delta (azodipyrrole beta-D-monoglucuronoside). Small amounts of azopigments with complex conjugating structures (gamma-azopigments) are present in both cases. Human bile further yields small amounts of azopigments containing xylose or glucose (called azopigments-alpha(2) and -alpha(3), respectively). Monoconjugated bilirubin (estimated from the percentage of azodipyrrole) amounts of 22% of total bile pigments in human bile and to 39% in murine bile. In both, the bulk of bile pigment is bilirubin diglucuronoside. From bile of patients with acquired liver diseases a new azopigment group (beta-azopigment) was derived. The gamma-azopigment group was increased; the delta-azopigment group (containing azodipyrrole beta-D-monoglucuronoside) was decreased. No differentiation was possible between intra- and extrahepatic cholestasis. The percentage of beta-azopigment showed a positive correlation with serum bilirubin concentration (r = 0.6). Recovery of the diseases was accompanied by normalization of the azopigment patterns. In rats, hydrostatic or mechanical obstruction induced increases in beta- and gamma-azopigments and a decrease in delta-azopigment similar to the changes observed in bile of liver patients. Complete normalization was obtained 6 hr after relieving the hydrostatic obstruction (duration 15-21 hr). In contrast, with man after surgery for extrahepatic obstruction, T-tube bile was not normalized when the T-tube was withdrawn (10 days after operation). Hydrostatic obstruction in rats provides an easy model when postobstructive bile pigment composition and parameters have to be investigated. The present investigations stress the importance of the physiopathological state when studying bilirubin conjugation. Hindrance to bile secretion induced heterogeneity of bilirubin conjugates and stimulated the formation of complex structures.