A phase 1 study of 42 non-human immunodeficiency virus type 1 (HIV)-infected volunteers was initiated to determine the safety and immunogenicity of an HIV subunit vaccine consisting of recombinant envelope gp120 derived from HIVSF2 (rgp120SF2) combined with a novel adjuvant, MF59, with or without the immunomodulator muramyl tripeptide dipalmitoyl phosphatidylethanolamine (MTP-PE). All injections contained adjuvant MF59, and subjects were grouped according to MTP-PE dose. Injections were given on days 0, 30, 180, and 365. The vaccine was well tolerated with limited local and systemic reactions. These immunizations induced rgp120SF2-specific binding antibodies that persisted > or = 24 weeks. After three immunizations, all subjects receiving the antigen developed neutralizing antibodies to HIVSF2, and serum from 67% of these subjects also cross-neutralized HIVMN. ELISA-reactive antibodies to the HIVSF2 V3 region and strong lymphoproliferative responses to HIVSF2 envelope proteins were detected in all rgp120SF2-immunized subjects.