Interferon-alpha (IFN-alpha) has been reported to be beneficial in the treatment of chronic active hepatitis occurring as a result of hepatitis B virus (HBV) infection. Treatment with IFN-alpha has been proposed as a means of reducing the high rate of allograft infection in clinical liver transplantation in patients transplanted for HBV-related chronic active hepatitis and cirrhosis who are positive for hepatitis B surface antigen (HBsAg). We obtained resected whole livers from two groups of patients who received liver transplants. Group A consisted of 11 patients who were HBsAg+ but were not treated with IFN-alpha, and group B consisted of 10 patients who were also HBsAg+ but received IFN-alpha therapy for 29.4 +/- 5.6 days prior to orthotopic liver transplantation. No differences between the two groups existed in terms of a variety of demographic and clinical characteristics. The liver tissue was stained with monoclonal antibodies to cell surface antigens unique to different mononuclear cell populations by the avidin-biotin-immunoperoxidase technique to determine the effect of IFN-alpha on the lymphocyte subsets as well as HLA antigen expression on liver-infiltrating mononuclear cells. The number of HLA-DR+ lymphocytes in the liver was significantly increased (P less than 0.005) within the portal areas in group B compared with that found in group A (84 +/- 14 versus 33 +/- 5 per one high-power field). Moreover, the intensity of the HLA-DR antigen expression on lymphocytes in the portal areas (P less than 0.02) and in the hepatic lobule (P less than 0.05) was greater in group B than in group A. The number of natural killer (NK) cells was increased in the portal areas (P less than 0.05) of group B compared with group A. These alterations in the lymphocyte and NK cell populations present in the liver in response to IFN-alpha therapy presumably reflect an IFN-alpha-induced enhancement of the immune response to virus-infected cells.