The documented association between the hepatitis-associated antigen (HAA) and long-incubation serum hepatitis has inspired a study of immunoglobulin patterns in serum during the course of disease in 65 hepatitis patients with demonstrable HAA and in 40 histopathologically verified cases of hepatitis without demonstrable HAA in acute-phase serum. The HAA-negative patients showed markedly increased levels of IgM that were significantly higher than those of the HAA-positive patients during the first month after onset of jaundice. Both groups had elevated levels of IgG, but normal levels of IgA. These different patterns, which may be of diagnostic help in selected cases of viral hepatitis, support the concept that viral hepatitis associated with HAA has an immunologic nature different from that of viral hepatitis not associated with HAA.