A peptide of formula (I):X-A-B-C-Trp-D-YwhereinX represents hydrogen, or a terminal nitrogn protecting group selecting from the group consisting of the acyl, aromatic oxycarbonyl, alkyl, aralkyl and aliphatic oxycarbonyl;A represent a valence bond or a L-.alpha.-amino acid residue;B represent a L-.alpha.-imino acid residue or a L-.alpha.-amino acid;C represent a L-.alpha.-imino acid residue or a neutral L-.alpha.-amino acid residue;D represents a valence bond or a L-.alpha.-amino acid residue; andY represents hydroxy, an amino group or a group of the formula OR, NHR, NR.sub.2 or NH--NH--R', wherein R represents a straight chain, branched chain or cyclic (including fused or bridged ring) alkyl group having up to 11 carbon atoms, optionally being substituted; a phenyl group or an aralkyl group having from 7 to 9 carbon atoms; and R' represents hydrogen, any of the groups which R may represent; a straight chain, branched chain or cyclic aliphatic acyl group having from 1 to 11 carbon atoms, optionally substituted by hydroxy, an amino group or a halogen atom; an aromatic acyl group, optionally substituted by hydroxy, amino, or a halogen atom; a straight chain, branched chain or cyclic aliphatic oxycarbonyl group having from 3 to 11 carbon atoms, or an aromatic oxycarbonyl group.