To analyze interleukin-2-dependent immunoregulatory function in hepatitis B virus infection and in other forms of viral hepatitis, levels of soluble interleukin-2 receptors (sIL-2R) were measured by an enzyme-linked assay in sera from patients with acute and chronic viral hepatitis of different etiology. Increased sIL-2R levels were detected in the early phase of acute hepatitis type A and type B, but not during acute non-A, non-B hepatitis. Among 46 patients with chronic hepatitis B virus infection, levels of sIL-2R were significantly increased only in cases with chronic active hepatitis, while they were about normal in chronic persistent hepatitis or in healthy carriers of the infection. These differences were independent of virus replication, being maintained when patients were stratified according to HBeAg/anti-HBe status and to serum HBV-DNA. Nine patients with chronic active hepatitis type B and high sIL-2R levels at presentation were followed prospectively for two to eight years, and in HBeAg-positive patients, the behavior of receptor levels closely paralleled disease activity. These results, which may reflect increased shedding of IL-2R by activated T lymphocytes in patients with active destruction of HBV infected hepatocytes, indicate the usefulness and potential prognostic importance of serum sIL-2R determination in patients with chronic viral hepatitis. Patients with chronic non-A, non-B hepatitis had much lower sIL-2R levels, although their liver disease was similar to hepatitis B cases, suggesting that different pathogenetic mechanisms operate in these patients.