Tests by counter-immunoelectrophoresis for antibody to hepatitis B core antigen (anti-HBc) were introduced into a routine testing programme for evidence of hepatitis B virus infection. Samples tested for anti-HBc were selected on the basis of the results of tests for HBsAg and clinical details. The sensitivity and specificity of the test were assessed and correlations made with the presence of HBsAg. The presence of anti-HBc was very useful in the interpretation of a doubtful positive result for HBsAg in the haemagglutination test. With very few exceptions the serum samples positive for HBsAg by routine tests also contained anti-HBc. It is concluded that the test is valuable and merits introduction into routine testing programmes.