The present invention provides novel polypeptides of A1-Arg-A2-Cys-Tyr-A3-A4-X-A5-A6-Cit Cys-A7 (I) or their salts (wherein A1 is hydrogen or a residue of arginine, lysine, ornithine, citrulline, alanine, or the like; A2 is an aromatic amino acid residue; A3, A4 and A6 are each a residue of arginine, lysine, ornithine, citrulline, or alanine; A5 is a residue of tyrosine, phenylalanine, alanine, naphthylalanine, or citrulline; A7 is a lysine or arginine residue whose carboxyl group may be converted into amido; and X is a residue of D-ornithyl-proline, prolyl-D-ornithine, D-lysylproline, or the like, with the proviso that any one of A1, A3, A4, A5, A6 and A7 is a residue of alanine or the like or that X is citrulline or the like), and methods of using same in the treatment of HIV.