The nature of the immunodominant a region of hepatitis B virus surface antigen (HBsAg) has been examined by mutation of specific amino acid residues. Proline 142 was required for the exhibition of full antigenicity. Replacement of cysteines 124 and 147 by serines drastically reduced or eliminated reactivity with antibodies to HBsAg, which implicates these two residues in stabilising the conformation of the antigen. The a region of HBsAg has also been shown to influence both the immunoreactivity of the adjacent subtype antigenic region, despite being immunologically distinct from it, and the ability of the antigen to interact with a subtype-specific monoclonal antibody. These results emphasise the importance of the polypeptide region between cysteine residues 124 and 147 in determining a antigenicity, as well as manifestation of the subtype of HBsAg, and reinforce the view that these are conformational epitopes.