Abstract: Methods and compositions are provided for the cloning and expression of plasmids bearing genes encoding a novel protein derived from HTLV-III. This protein, which is called the gag/env protein and which contains antigenic determinants from both the core and envelope proteins of HTLV-III, can be purified to homogeneity and used as the basis for diagnostic tests to detect the presence of antibodies against viruses associated with AIDS or the viruses themselves in human sera and other biological fluids. The gag/env protein may also be formulated for use as a vaccine for protection against AIDS through prophylactic immunization.