Novel tridecapeptides having the following amino acid sequence:H-Tyr-X-Gly-Phe-Met-Thr-Ser-Glu-Lys-Ser-Gln-Thr-Pro-Ywherein: X is a chiral residue of a D-amino acid selected from the group consisting of D-alanine, D-leucine, D-isoleucine, D-valine, D-phenylalanine, D-tyrosine, D-tryptophan, D-serine, D-threonine, D-methionine, D-glutamic acid, D-glutamine, D-proline D-aspartic acid, D-asparagine, D-lysine, D-arginine and D-histidine; and Y is selected from the group consisting of hydroxy, amino, loweralkylamino, dilowerakylamino and lower alkoxy; and the pharmaceutically acceptable salts thereof; intermediates useful in making the novel compounds; and pharmaceutical compositions and methods employing the novel compounds.