Disclosed is a penicillin derivative of the formula: ##STR1## wherein n is an integer of 0, 1 or 2; Y is a cyano group, a lower acyl group, a mono- or di-lower alkylthiocarbamoyl group, ##STR2## wherein R.sub.1 is a hydrogen atom, a lower alkyl group, a phenyl group, a group --(CH.sub.2).sub.m --OR.sub.2 or --(CH.sub.2).sub.m COOR.sub.2 (m is an integer of 1 to 6 and R.sub.2 is a hydrogen atom or a penicillin carboxyl ester-forming group which is commonly used for penicillin derivatives) or a phenyl group substituted by at least one member selected from the class consisting of lower alkyl group, halogen atom and lower alkoxy group; and R is a hydrogen atom or a penicillin carboxyl ester-forming group, or a salt thereof. They are useful as .beta.-lactamase inhibitors.