B-cell differential factor (BCDF) activities were determined in 58 patients with various types of hepatitis B. In comparison with normal subjects, BCDF activities were significantly increased in patients with FH and CAH (P less than 0.01), markedly decreased in HBsAg carriers (P less than 0.01) and presented no change in patients with CPH and AH. It is interesting that there was a significant positive correlation between BCDF activity and serum titer of anti-HBC or serum globulin levels. No correlation was observed between BCDF activity and serum anti-HBs levels. It is suggested that abnormal BCDF activity might attribute to aberration of immunoregulation of HBV infections.