A peptide derivative of the formula: wherein A is a group of the formula R11O-CO-(wherein R11 is a lower alkyl group or a phenyl group), or a group of the formula R12R13N-C(=O)-(wherein R12 is a lower alkyl group, a cycloalkyl group, a 1-adamantyl group, a phenyl group wherein one or two optional hydrogen atoms on the benzene ring may independently be replaced by a halogen atom, a trifluoromethyl group, a nitro group, an amino group or a formylamino group, a pyridyl group, or a thienyl group, R13 is a hydrogen atom, a lower alkyl group or a cycloalkyl group, or R12 and R13 form, together with the adjacent nitrogen atom, a 5-to 9-membered nitrogen-containing saturated heterocyclic ring having 4 to 8 carbon atoms, wherein among methylene groups forming the ring, one optional methylene group not adjacent to the above nitrogen atom may be replaced by a thio group, and one to four optional hydrogen atoms on the carbon atoms of the heterocyclic ring may independently be replaced by a lower alkyl group, and further two adjacent carbon atoms in the heterocyclic ring may form a benzo-fused ring); B is an oxygen atom or a group of the formula-NR2-(wherein R2 is a hydrogen atom or a lower alkyl group); R3 is a lower alkyl group, a cycloalkyl group, an aryl group, a heterocyclic group, a cycloalkyl lower alkyl group, an aryl lower alkyl group or a heterocyclic lower alkyl group; X1 is an oxygen atom or a group of the formula-NR?-(wherein R? is a hydrogen atom or a lower alkyl group); R? is a 3-indolylmethyl, 3-benzothienylmethyl, 1-naphthylmethyl or benzyl group wherein one or two optional hydrogen atoms on the ring may be replaced by a hydroxyl group, a halogen atom, a formyl group, a lower alkyl group, a lower alkoxy group, a lower alkylthio group, a lower alkylsulfinyl group, a lower alkylsulfonyl group, a lower alkoxycarbonyl group, a nitro group or a group of the formula R?1-CO-X2 {wherein R?1 is a lower alkyl group, a lower alkoxy group, or an amino group which may be substituted by a lower alkyl group, and X2 is an oxygen atom or a group of the formula-NR?2-(wherein R?2 is a hydrogen atom or a lower alkyl group)}; X3 is an oxygen atom or a sulfur atom; R? is a hydrogen atom, or a lower alkyl or lower alkenyl group which may have a substituent selected from the group consisting of a hydroxyl group, a lower alkoxy group, a lower alkylthio group and a heterocyclic group; n is 0 or 1; Y is a hydroxymethyl group, a group of the formula CO? R?1 (wherein R?1 is a hydrogen atom or a lower alkyl group), a group of the formula CONHR?2 (wherein R?2 is a hydrogen atom or a lower alkyl group which may have a substituent selected from the group consisting of a hydroxyl group, a carboxyl group and a sulfo group), a 1H-tetrazol-5-yl group, a sulfo group and a phosphono group; or a pharmaceutically acceptable salt thereof.