A method for stimulating an immune response in an animal, said method comprising administering to said animal an effective amount of an immunogenic composition comprising a retroviral superantigen peptide, wherein said superantigen peptide is a peptide selected from the group consisting of a peptide designated HIV-1 Nef(1-38) consisting of an amino acid sequence shown in SEQ ID NO. 14 and a peptide designated HIV-1 Nef(123-160) consisting of an amino acid sequence shown in SEQ ID NO. 18, wherein said immune response is selected from the group consisting of antibody production by B lymphocytes, proliferation of CD4.sup.+ lymphoctes, the production of IL-2, and the production of IFN-.gamma..