Cyclic peptides comprising, as a constituent chain or chains, one or two amino acid sequences selected from the groups consisting of the amino acid sequence Glu-Ala-Asp-Asp-Arg and the amino acid sequence Ser-Gln-Lys-Glu-Gly, and AIDS vaccines containing the cyclic peptide as an active ingredient. Preferably a cyclic dodecapeptide represented by the formula given below and an AIDS vaccine containing the cyclic dodecapeptide as an active ingredient. From the in vivo absorption and antibody formation viewpoint, active groups selected from among the carboxyl, amino and hydroxyl groups contained in the cyclic peptide is preferably bound to substituent groups. The cyclic dodecapeptide can neutralize the second receptors in the infection of human with HIV-1 virus ##STR00001##