Hepatitis A virus (HAV) transmission through blood is a rare but potential cause of posttransfusion hepatitis. We can now document such a case supported by laboratory evidence of HAV in the donor blood. A 10-year-old girl manifested icteric hepatitis A 31 days after receiving a single unit of packed RBCs from a donor who subsequently experienced hepatitis A and died in hepatic failure. Hepatitis A virus antigen was detected in the donor's hepatocytes and in plasma obtained from the original donor unit. The density in cesium chloride of the HAV antigenic activity from the liver and plasma ranged from 1.33 to 1.37 g/cu cm, which is similar to that reported for infectious HAV particles. The implicated donor plasma had normal aminotransferase levels and was negative for antibody to HAV. Inoculation of this plasma into a chimpanzee resulted in the development of hepatitis A 23 days later based on the appearance of fecal HAV antigen, hepatitis, and IgM anti-HAV seroconversion. These data clearly document the presence of HAV in the donor sample that produced posttransfusion hepatitis A.