Of 1,801 Chinese subjects, age 1 to 90 years, screened for hepatitis B surface antigen and antibody (HBsAg, anti-HBs) and antibody to hepatitis B core antigen (anti-HBc), 214 (11.9%) had an isolated, positive anti-HBc result; anti-HBc was reproducibly present in the initial sera in only 66% and persisted after an interval of 2 weeks to 3 months in only 73%. There was a strong correlation between the rates of reproducibility and persistence of isolated anti-HBc and the initial anti-HBc titers. Thirty-two subjects with persistent, isolated anti-HBc received four doses of hepatitis B vaccine (5 micrograms, HEVAC B) at 0, 1, 2 and 12 months: 56% developed a primary anti-HBs response in response to hepatitis B vaccine, 16% developed an anamnestic or secondary anti-HBs response, and 28% were undetectable for anti-HBs even after four doses of vaccine. The low rates of reproducibility and persistence of anti-HBc together with the high rate of primary anti-HBs response to hepatitis B vaccine in subjects with isolated anti-HBc raise doubts as to the reliability of anti-HBc (Corzyme, Abbott Laboratories, North Chicago, Ill.) as a single screening test for hepatitis B infection prior to vaccination and suggests that subjects with isolated anti-HBc, in particular those with low anti-HBc titers, be included in vaccination programs.