The bile acid pattern was determined in 12 surgical biopsy specimens, and in four necropsy specimens of human liver. In normal livers, cholic acid is usually the predominant one. Only traces of lithocholic acid were found. In severe biliary obstruction, cholic acid rose conspicuously, whereas chenodeoxycholic acid rose only when cholestasis was protracted. In the patient with the most prolonged jaundice, chenodeoxycholic acid exceeded cholic acid. Lithocholic acid and bile acid sulfate esters comprised only a minor fraction of the total bile acids in cholestasis. The sum of deoxycholic acid and chenodeoxycholic acid parallels the degree of feathery degeneration as an expression of cholestatic liver cell injury. It is assumed that the rise of cholic acid, which does not appreciably interact with the microsomal biotransformation system in contrast to the dihydroxylated bile acids, represents a transient protective mechanism in human extrahepatic biliary obstruction.