Composition and method for detection of and vaccination against the viral causative agents of AIDS and ARC. The composition is a synthetic peptide, the amino acid sequence of which is sufficiently homologous to the amino acid sequence of the gp 41 and gp 120 subunits of the gp 160 envelope glycoprotein of HTLV-III, LAV or ARC to produce an immunogenic response in a patient and which has a hydrophilic region therein. The composition may be conjugated to an appropriate carrier for use as a vaccine against AIDS and ARC or bound to the ligand of a specific binding pair, contacted with a sample of biological fluid from a patient suspected of having contracted AIDS or ARC and thereafter contacted with the anti-ligand of the specific binding pair for detection of any antibodies against the viral causative agents of AIDS or ARC which may be present in the sample.