Abstract: Peptides are derived from the envelope glycoprotein of HIV virus and having one of the following formulae ##STR1## in which X represents an NH.sub.2 group which is free or converted into an amide by one or two alkyl groups comprising from 1 to 5 carbon atoms, and Z represents either an OH group which is free or present as an alkoxy and thereby containing an alkyl group comprising from 1 to 5 carbon atoms, and in which at least one of the Cys residues is deleted, substituted or protected.The peptides are applied to the detection of infection caused by the viruses HIV-1 and/or HIV-2 and to the vaccination against AIDS.