AbstractAn outbreak of hepatitis A occurred in a rural elementary school in Colbert County, Alabama, in October and November 1972. The outbreak was caused by sewage‐contaminated drinking water and involved 49 children and 1 adult. Acute and convalescent serum samples were obtained from the hepatitis patients, and from each of 24 unafflicted matched controls at the same time. Strikingly, the levels of the third and fourth components of complement (C'3 and C'4) were markedly reduced in the acute samples from the sick children but returned to normal in the convalescent ones; the C'3 and C'4 levels in the healthy matched controls were not reduced. In addition, antigen‐antibody complexes were found in convalescent sera from some of the sick children. These findings indicate that immune complexes are present in acute hepatitis A and suggest that the techniques for future virus isolation may have to be alte