Serum endogenous interferon-alpha was determined in 42 patients with chronic hepatitis B virus infection by radioimmunoassay using an Abbott kit (USA). The study sample included 26 males and 16 females, aged 3 to 59 years (mean, 30.1). Ten of these patients had a history of acute viral hepatitis B carrying HBsAg for more than 6 months. Thirty two patients were accidentally found to be virus carriers for 8 months to 15 years. Six of these were treated with interferon-alpha and one with Ursofalk. Forty one patients (97.62% +/- 2.38) were anti-HBcIgG positive which confirmed former hepatitis B virus infection. Only one patient who was anti-HBcIgG positive was found to be also anti-HBcIgG positive, anti-HBcIgM positive, HBeAg negative, and anti-HBe positive later biophysically verified as exacerbated chronic active hepatitis. Serum HBeAg was detected in 13 of the whole sample (30.95% +/- 7.13); 8 patients (32% +/- 9.33) were HBV-DNA positive, i.e., a third of the cases presented with active replication of the hepatitis B virus. In all 42 patients with chronic hepatitis B virus infection (convalescence and health HBsAg carriers) serum interferon-alpha levels were nil or close to nil. Only in the patient with chronic active hepatitis the serum interferon level was 3.83 IU/ml. These data support the observations that interferon-alpha production is reduced in chronic hepatitis B virus infection and are consistent with the view that treatment with exogenous interferon-alpha stimulates the clearance of the virus.