Fragments of the human immunodeficiency virus (HIV) envelope coding region have been fused with the hepatitis B virus envelope middle protein. In this system, HIV antigenic determinants are exposed at the surface of a highly antigenic structure, the hepatitis B surface antigen particle. Immunization of rabbits with these particles elicited antibodies directed against both parts of the hybrid protein. One of the rabbit antisera not only exhibited a neutralizing effect on the original HIV1 isolate but also on a divergent Zairian isolate. The HIV sequence in this recombinant is 84 amino acids long and contains conserved and variable domains and a region critical for interaction with the CD4 receptor. Such recombinant antigens could be primary elements in the design of a polyvalent vaccine.