Serological analysis by radioimunoassay of sera from 297 patients hospitalized with acute non-toxic hepatitis was used for classification according to virus etiology. Radioimmunoassays included tests for hepatitis B surface antigen (HBsAg), antibody to HBsAg (anti-HBs), antibody to hepatitis A virus (anti-HAV), anti-HAV of IgM class, and antibody against cytomegalovirus (GMV) and Epstein-Barr virus. One patient with a significant rise in anti-CMV antibodies was classified as having CMV hepatitis. Among the 296 remaining patients serological markers indicated hepatitis A in 51 cases (17.2%) and hepatitis B in 208 cases (70.3%). The remaining 37 patients (12.5%) fulfilled criteria for acute non-A, non-B hepatitis. This type of hepatitis had symptoms and signs indistinguishable from those of hepatitis A, except for a slight tendency to milder disease on admission. A considerable proportion of patients with non-A, non-B hepatitis had a history of drug abuse (43.2%) and of recently traveling to endemic hepatitis areas (29.7%). In the remaining 27.1% no particular background was revealed. No case of post-transfusion hepatitis was seen. During the last 6 months of the study a striking change in epidemiology concerning hepatitis A was seen, apparently caused by a steep increase in the incidence of this type of hepatitis among drug addicts. No significant difference in biochemical liver tests was seen in non-a, non-B hepatitis or hepatitis A. In contrast, a marked and statistically significant difference in serum concentrations of IgM was found, with higher values (mean, 7.5 g/1; range, 3.2-13.9 g/1) in hepatitis A than in non-a, non-B hepatitis (mean, 3.3 g/1; range, 0.9-9.4 g/1). This difference may have diagnostic value.