It has been suggested that the quantitative estimation of serum alpha-1-fetoprotein may help in distinguishing the neonatal hepatitis syndrome from biliary atresia. We measured the serum AFP concentration in 52 neonates and infants with various hepatobiliary disorders, including neonatal hepatitis syndrome (group I), biliary atresia (group II), and other hepatopathies such as choledochal cyst (group III). The mean serum AFP concentration in patients with neonatal hepatitis was significantly greater than the mean concentration in the other two groups. There was no significant difference between the mean serum AFP concentrations in patients with biliary atresia and in group III patients. Patient age was noted to be an important factor: Serum AFP levels greater than 35 microgram/ml in infants one to four months of age suggpst the diagnosis of neonatal hepatitis syndrome. Serum AFP levels below 10 microgram/ml in infants less than four months of age suggest the diagnosis of biliary atresia or hepatopathies other than neonatal hepatitis. However, the variable and significant overlapping of serum AFP values between 10 and 35 microgram/ml limit the diagnostic value of this test.