Serum unconjugated and conjugated bile acids in young infants with intrahepatic cholestasis (idiopathic neonatal hepatitis syndrome, n = 8) or extra-hepatic cholestasis (preoperative extrahepatic biliary atresia, n = 8) were examined by an enzymatic procedure and high-performance liquid chromatography. In comparison with the mean level of total serum bile acid of controls having no liver or gastrointestinal diseases, those of each group markedly increased (15.6 +/- 5.1 vs. 120.9 +/- 64.0 and 161.8 +/- 54.2 nmol/ml), but those of unconjugated bile acid were almost unchanged (1.4 +/- 0.5 vs. 1.0 +/- 0.6 and 0.6 +/- 0.2 nmol/ml). The ratios of cholate to chenodeoxycholate and glycine- to taurine-conjugated bile acids (G/T) were not significantly different between the groups of intrahepatic and extrahepatic cholestasis. However, in the patients with intrahepatic cholestasis, the G/T ratio varied greatly and the quantitative determination of individual conjugated bile acids in serum revealed that a half of the patients examined had very low levels of taurine-conjugated cholate and chenodeoxycholate, suggesting a bile acid metabolism alternation specific for underlying intrahepatic cholestasis.