Human immunodeficiency virus (HIV) is the etiological agent of acquired immunodeficiency syndrome (AIDS). At present there is no effective vaccine against this disease. The objects of this project are to characterize HIV antigens, determine the targets of humoral and cell-mediated immunity, and to use this information to develop candidate vaccines. We have constructed recombinant vaccinia viruses containing genetic information of HIV and the closely related simian immunodeficiency virus. These recombinant viruses have been used to prepare purified proteins, monoclonal antibodies (mAbs), targets for cytotoxic T cells, and candidate vaccines. We continued to characterize a large panel of mAbs that were produced against a novel soluble oligomeric form of the HIV-1 envelope protein. To identify and map conserved epitopes and regions in gp41 where structure is influenced by quaternary interactions, we used a panel of 38 conformation-dependent and 9 conformation- independent anti-gp41 mAbs. By cross competition experiments 6 distinct antigenic determinants were identified and mapped. Three of these determinants are conformational in nature and dependent in part on env oligomeric structure. mAbs to two were broadly cross-reactive with env proteins derived from primary virus strains. The prevalence of antibodies in HIV-1 positive human sera to the antigenic determinants was demonstrated by blocking the binding of mAbs to env proteins. The folding, assembly, and intracellular trafficking of the HIV-1 envelope glycoprotein were analyzed with mAbs recognizing maturational intermediates. Based on reactivities with gp160 at different times after pulse-labeling, the mAbs were sorted into groups that exhibited binding which was: immediate and constant, immediate but transient, delayed, late, or very late. Epitopes recognized by monomer-specific or CD4-blocking mAbs were present on gp160 molecules associated with the molecular chaperone BiP/GRP78. mAbs with preference for monomers reacted with recombinant or HIV-1 envelope proteins in the endoplasmic reticulum whereas the oligomer-specific mAbs recognized them in the Golgi complex. The highly attenuated and host-restricted modified vaccinia virus Ankra (MVA) strain was used as a vector to express the simian immunodeficiency virus (SIV) envelope and gag-polymerase genes. Rhesus macaques were immunized with the live recombinant viruses and then challenged with an uncloned, homologous, cell free SIV. All immunized macaques exhibited a sustained, marked reduction of virus load in plasma, peripheral blood mononuclear cells, and lymph nodes compared to control animals.