Hepatitis A and hepatitis B are viral infections of the liver. Hepatitis A is spread by the fecal-oral route--i.e., by ingestion of virus shed in the stool of acutely infected individuals. The virus is transmitted from person to person or (in outbreaks) via contaminated food or water. Population groups at increased risk of acquiring hepatitis A include children and staff in day care centers. Hepatitis B is spread by blood and other body fluids from acutely infected individuals or chronically infected carriers. Infection occurs when virus contained in these fluids enters the body through mucosal surfaces or breaks in the skin. A vaccine against hepatitis B has been developed. It consists of noninfectious hepatitis B surface antigen purified from the plasma of chronic carriers. The three sequential inactivation treatments used in manufacture of the vaccine kill hepatitis B virus and other infectious agents that may be present in human plasma, including the human T cell-lymphotropic virus that causes the acquired immunodeficiency syndrome. The vaccine is well tolerated, highly immunogenic, and highly effective in preventing hepatitis B. Both live attenuated and killed vaccines against hepatitis A are also being investigated. A live attenuated vaccine is preferred and seems feasible on the basis of initial studies in animals and volunteers.