Serum samples from 133 persons who were positive only for antibody to hepatitis B core antigen (anti-HBc) by enzyme immunoassay (EIA) were retested for seromarkers of hepatitis B virus (HBV) by radioimmunoassay and for HBV DNA by polymerase chain reaction analysis. All persons were subsequently vaccinated with hepatitis B vaccine. HBV DNA was found in only five persons, four of whom remained positive during retesting. Most persons had a primary antibody response with three doses of hepatitis B vaccine. Evidence of HBV DNA was not detected in 96% of persons with isolated anti-HBc by EIA.