Fourteen infants aged from 2 to 5 months were admitted to hospital with acute viral hepatitis. Their clinical presentation ranged from severe disease to fulminant hepatitis. In all patients the prothrombin-time was 10% or less of normal and serum glutamic pyruvic transaminase and bilirubin were increased. In eight cases liver-biopsy specimens were obtained during liver failure and showed a widespread necrosis without inflammatory cells. Hepatitis-B-surface antigen (HBSAg) and antibody (HBSAb) were sought by several techniques, including passive haemagglutination and radioimmunoassay. Hepatitis was associated with hepatitis-B virus in eleven out of fourteen patients as judged by the detection of HBSAg and/or a secondary rise in HBSAb. In eight cases, the infants had received blood-derivatives in the neonatal period. The mothers of five of the remaining cases were found to be chronic carriers of HBSAg. Despite intensive supportive therapy, including repeated exchange transfusions and administration of anti-HBS gamma-globulins (six cases), eight patients died. These cases demonstrate that severe or fulminant type-B hepatitis can develop in infants, who are capable of completely eliminating the hepatitis-B virus. They also suggest that severe hepatitis can result from maternal contamination.