Abstract: Novel and biologically useful peptides of the formula: A.sub.1 -A.sub.2 -L-Phe-X-L-Thr-L-Ser-R.sub.1 -Y-L-Ser-R.sub.2 -L-Thr-L-Pro-L-Leu-L-Val-L-Thr-B are disclosed (together with pharmaceutical compositions comprising a pharmaceutically effective amount of same) wherein:(a) A.sub.1 and A.sub.2 each represent an aminoacid residue of the formula H.sub.2 N-ALK-CO-, where ALK is an alkylidene group with 1 to 6 carbon atoms inclusively;(b) X is an amino-acid residue selected from the group consisting of L-Met, L-Met(O), L-Met(O.sub.2) and L-Leu;(c) R.sub.1 and R.sub.