Hepatitis B virus (HBV) DNA was tested for in 294 blood units which had antibody against hepatitis B core antigen (anti-HBc) as the isolated serological marker of HBV infection. After amplification by polymerase chain reaction, HBV DNA was detected in 12 (6.9%) of 175 units that were positive for anti-HBc with hemagglutination inhibition titers greater-than-or-equal-to 2(6), significantly more often than in none of 119 units with titers less-than-or-equal-to 2(5) (p < 0.01). These results indicate that the exclusion of blood units with isolated high-titer anti-HBc would be effective for further decreasing the risk of posttransfusion hepatitis B.