A technique has recently been described by Blanckaert and his colleagues that specifically and accurately quantifies unconjugated bilirubin, diconjugated bilirubin, and the C-8 and C-12 isomers of monoconjugated bilirubin. This technique has now been used to determine the distribution pattern of bilirubin and its ester conjugates in 91 sera from 65 patients with hepatobiliary disease, and the results were compared with two conventional diazo assays. Both diazo assays yielded values for total bilirubin concentration that were markedly and unpredictably higher than those obtained by the new technique, and the direct-reacting fraction by diazo assay showed little or no agreement with the fraction of total ester conjugates determined by the new method. Previous studies using the new method had shown that bilirubin conjugates are undetectable in sera from healthy adults or individuals with Gilbert's syndrome, but they were found in 89 of the 91 present patient sera. The fraction of total serum bilirubin represented by C-8 monoconjugates, C-12 monoconjugates, diconjugates, and total ester conjugates was higher in patients with biliary obstruction than in those with parenchymal liver disease, but extensive overlap between groups prevented determination of these conjugated species from being diagnostically useful. Overall, bilirubin ester conjugates in serum consisted of 30% C-8 monoconjugates, 37% C-12 monoconjugates, and 33% diconjugates, while urine contained predominantly diconjugates.