Cell-mediated immunity (CMI) to hepatitis B surface antigen (HBsAg) was investigated in patients who recovered from acute hepatitis B and in chronic carriers of the antigen, either with or without liver disease. The study was performed using leucocyte migration inhibition test and purified HBsAg. In antigen-negative post-hepatitis patients the frequency of cellular immunity to HBsAg (63.6%) showed a significant difference to controls. On the other hand, only one out of 5 antigen-positive post-hepatitis patients showed sensitisation to HBsAg; four out of these cases had chronic hepatitis at liver biopsy. In HBsAg-chronic carriers, with or without liver disease, the frequency of CMI to HBsAg was not significant to controls. A significant correlation was noted between inflammatory activity and inhibition of migration. Our results suggest that a normal cellular immune response to HBsAg is necessary for the recovery from acute hepatitis B and for the clearance of the virus. In HBsAg-chronic carriers this response seems to be absent or inadequate, leading to the state of healthy chronic carriers or to the evolution to HBsAg-chronic hepatitis, respectively. The high frequency of CMI in HBsAg-negative chronic active hepatitis, suggests that hepatitis B virus infection may play a role in the ethiology of the disease.