Vaccine-associated hepatitis B surface antigen (HBsAg) mutations have been mainly identified within the "a" determinant (aa 124-147). Now changes have been detected that are located outside the "a" determinant from immunized Singapore infants born to hepatitis B virus (HBV) carrier mothers. These include Asn116 to Thr116, Val118 to Ala118, Pro120 to Ser120, Ala159 to Val159, Phe183 to Cys183, and Val184 to Ala184. Decreased binding to "a" determinant-specific monoclonal antibody was observed for variants Pro120 to Ser120, Ala159 to Val159, and Phe183 to Cys183. Unlike other HBsAg variants, the Asn116-to-Thr116 HBsAg variant had the wild type threonine in the infant, whereas the mutated asparagine was found in the mother. Vertical transmission is indicated for Phe183-to-Cys183 and Val184-to-Ala184 HBsAg variants, as they were found in both the infants and mothers. Detailed analysis of these HBsAg variants would provide further understanding of the antigenic structure of HBV.