Single liver biopsies from 102 clinically diagnosed hepatitis patients were examined by immunofluorescence for the presence of hepatitis B surface antigen (HBsAg) and hepatitis B core antigen (HBcAg), complement and immunoglobulin deposition, and for their capacity to fix human complement in vitro. Of the sixty-five HBsAg positive livers, fifty-three were histologically diagnosed as chronic hepatitis, three as acute hepatitis, five as acute hepatitis with signs of transition to chronicity, and four as 'near normal liver'. In the group with chronic hepatitis, HGcAg was observed in thirty-nine livers, all of which also had HBsAg. Thirty-five of these thirty-nine cases also had the ability to fix complement in vitro in the hepatocyte nuclei and/or cytoplasm. Of these thirty-five cases, twenty-nine were positive for immunoglobulin deposition on the nuclei. All of these cases had antibody to HBcAg in the blood, but only five had anti-HBs. The frequency of in vitro complement fixation and immunoglobulin deposition was higher in active forms of the disease, such as chronic aggressive hepatitis and active cirrhosis, than in non-active disease such as chronic persistent hepatitis and mild cirrhosis. By the application of double fluorescent staining techniques, complement fixation was observed in some HBcAg-positive nuclei. In the 'near normal liver' cases there was no intrahepatic accumulation of HBcAg, and despite the presence of anti-HBc in the blood, in vitro complement fixation and immunoglobulin deposition were both absent. The group of three HBsAg ositive 'acute hepatitis with signs of transition to chronicity' cases behaved similarly to those with chronic aggressive hepatitis and had circulating anti-HBc, in vitro complement fixation and immunoglobulin deposition in the hepatocytes. None had circulating anti-HBs. In the group sith HBs-positive acute hepatitis, anti-HGc in the blood was the only other evidence of hepatitis B virus infection.