Hepatitis B virus associated DNA polymerase activity, hepatitis b surface antigen (HBsAg), and serum aspartate aminotransferase were followed in 21 patients with chronic active hepatitis while immunosuppressive therapy (prednisone +/- azathioprine) was being withdrawn. In every case, DNA polymerase activity fell within 6-10 wk of decreasing treatment and became undetectable in 8 patients. This was usually accompanied by a fall in HbsAg titer and a transient rise in serum aspartate aminotransferase activity. Four additional patients with previously untreated HbsAg positive chronic active hepatitis were placed on prednisone for 12 wk. There was a rise in DNA polymerase activity and HBsAg titer with a fall in serum aspartate aminotransferase values during treatment. Upon discontinuing therapy, DNa polymerase activity fell dramatically in all 3 patients who completed their course of prednisone and became undetectable in 1. These findings suggest that immunosuppressive therapy has a potentiating effect on hepatitis B viral replication in patients with chronic active hepatitis.