Imunoglobulin(Ig)-containing plasma cells in human liver were investigated in different aetiological and histological forms of acute hepatitis. Liver specimens from 93 patients with acute hepatitis (A, B, non-A, non-B and drug-induced) were studied by conventional microscopy and by immunoperoxidase staining for IgG, IgA and IgM in paraffin sections. Plasma cells were found in 78 of 93 specimens and Ig-containing cells in 75. IgG-containing cells were significantly more abundant in acute hepatitis with bridging necrosis than in other forms (p less than 0.01), and there were more IgA-containing cells than in the classical and periportal forms (p less than 0.05). IgG-containing cells were more numerous in acute hepatitis with periportal or panacinar necrosis than in the classical form (p less than 0.05). IgG- and IgA-containing cells were less abundant in drug-induced hepatitis than in viral hepatitis (p less than 0.05). Among the viral groups the largest number of IgG-containing cells was found in non-A, non-B hepatitis, while IgA-containing cells were most abundant in type A. These differences were not statistically significant. IgM-containing cells were very scanty in all groups studied. It is concluded that Ig-producing plasma cells are common in acute viral hepatitis and presumably play a part in antibody-dependent immune reactions in this disease.