Antibody to the d and y subtypes of hepatitis B antigen (HB Ag) was detected by a modification of the passive hemagglutination assay (PHA) for anti-HB Ag. With this technique, anti-d or anti-y activity could be detected in many sera negative for anti-HB Ag by counterelectrophoresis but positive by PHA. The proportion of sera with detectable anti-subtype antibody increased as the PHA titer increased. Studies of blood donors from the United States and Egypt revealed frequencies of each anti-subtype antibody which closely paralleled the frequency of the same antigen subtype in each population. In all instances of antibody sero-conversion in which anti-subtype antibody could be detected, the antibody was directed against the same subtype as the antigen associated with infection or immunization. Antibodies against both subtypes of antigen were not detected in any patient, including those who had been repeatedly transfused.