Serum glycocholic acid (SGC) was measured by radioimmunoassay in 277 samples from 122 children with hepatobiliary disorders and from 23 healthy age-matched controls. In patients with hepatobiliary disease the SGC was more frequently abnormal (83%) than values for serum albumin (7%), prothrombin time (17%), bilirubin (22%), alkaline phosphatase (45%), aspartate transaminase (57%) and gammaglutamyl transpeptidase (63%). The cumulative frequency of abnormality of these six tests was equal to that of SGC alone. Serum glycocholic acid concentrations were raised in 13 patients in whom all other tests of liver function were normal. Two of these had clinical and histological evidence of liver disease, while four had biopsy-proven hepatic fibrosis or cirrhosis, and two of three with chronic active hepatitis in remission subsequently relapsed. Four patients have as yet, no other clinical or biochemical evidence of continuing liver disease. Serum glycocholic acid was normal in seven children with abnormal aspartate transaminase or gammaglutamyl transpeptidase in whom there is strong suspicion of significant hepatic disease. A wide range of values of SGC was found with marked overlap between the values found in the different disease entities studied. The SGC value was related to the serum concentration of aspartate transaminase and gammaglutamyl transpeptidase but not to other tests of liver function. Serum glycocholic acid concentration was considered in relation to the severity of histological abnormality in 25 percutaneous liver biopsies. The extent of the rise in SGC was related to the presence or degree of histological severity of oedema in the portal tracts, disruption of the limiting plate, parenchymal fibrosis and hepatocellular necrosis but not to other histological features. The very high incidence of abnormal SGC values found in this study does suggest that in an ordinary inpatient and outpatient service SGC determination is a practical and sensitive indicator of the presence of significant liver disease but for its comprehensive identification aspartate transaminase and gammaglutamyl transpeptidase must also be determined.