In a survey of hepatitis B virus (HBV) subtypes using one set of monoclonal typing antibodies, 96.6% of the samples were ad or ay and 3.3% (31) were untypeable. Using two additional antibody panels, 23 samples were ay and ad. Six samples required a third panel to be typed as ay. One sample was not typeable. These last 7 samples were nonreactive or had low reactivity with some antibodies to the common a determinant. Sequencing the HBsAg gene of these 7 samples revealed that amino acid (aa) 122 was arginine, as expected for the y determinant. However, mutations giving rise to serine at aa 120, leucine at aa 143, and glutamic acid at aa 144 were found singly or in combination and correlated with the monoclonal antibody binding patterns. These results have implications for further subtyping surveys, vaccines, immunotherapy, and the design of diagnostic assays, and they give clues to the structure of the major neutralizing epitope of HBV.