Anti-HBc, the antibody to core antigen of hepatitis B virions, was assayed by a new solid phase sandwich radioimmunoassay inhibition method in the sera of 26 patients with HBsAg-negative chronic active hepatitis (CAH) and 31 patients with primary biliary cirrhosis (PBC). The sensitivity of the method was validated by finding anti-HBc titers averaging greater than 1:1000 in a group of 8 chronically HBsAg-positive individuals, 4 of whom had chronic persistent hepatitis and 4 of whom had no histological or biochemical evidence of liver disease. Anti-HBc was not detectable in any of the 26 patients with HBsAg-negative CAH. Sera from 2 of the 31 PBC patients contained anti-HBc but only in low titers, a distribution of anti-HBc similar to that found among a comparison group of medical personnel. Anti-HBc testing among PBC patients and control subjects identified a few persons in whom negative tests for HBsAg and anti-HBs failed to identify previous hepatitis B infections. The uniformly negative tests for anti-HBc among the HBsAg-negative patients with CAH provide additional evidence that this subgroup, typically young-middle age females, seldom derive CAH from hepatitis B infection.