CKS-17 peptide corresponds to aa 533-549 of the murine leukemia virus (MLV) envelope protein, the immunosuppressive region present in any retroviral envelope proteins. Synthetic peptides derived from this region inhibit immune function in vitro and in vivo.H-Leu-Gln-Asn-Arg-Arg-Gly-Leu-Asp-Leu-Leu-Phe-Leu-Lys-Glu-Gly-Gly-Leu-OH or H-LQNRRGLDLLFLKEGGL-OH