Hepatitis B surface antigen (HBsAg) has been reconstituted with different phospholipid classes. All epitopes defined by a panel of monoclonal antibodies which recognize both group- and subtype-specific antigenic determinants showed specificity for acidic phospholipids. Electrostatic interactions between HBsAg proteins and acidic phospholipids are partly responsible for the complete recovery of the antigenic properties. In addition to the nature of the polar head group, the fatty acid composition of the phospholipid also influenced the recovery of the antigenic activity. Negatively charged phospholipids must bear at least one unsaturated fatty acid in order to be effective in recovering full antigenic activity of HBsAg. The results reported herein support the conclusion that the antigenic activity is dependent on the physical state of the phospholipid moiety. The appropriate membrane fluidity is required for optimum conformation but, once this conformation is established, additional interactions imparted by the various phospholipids give a difference in the patterns of antigenicity. The analysis of binding of the monoclonal antibodies allowed the classification of the epitopes into two groups according to their dependence on the lipid moiety. Of all the antigenic determinants only those close to the lipid-protein interface would change upon direct interaction with the phospholipids. The rest would depend on the correct protein conformation determined by the appropriate phospholipid composition.