Abstract: .beta.-Lactam compounds represented by the general formula: ##STR1## (wherein X represents CH.sub.2, S or O; Y represents a hydroxyl group, a lower alkanoyloxy group or a lower alkoxycarbonyloxy group; W is a group represented by CH.sub.2, NH, ##STR2## A is a phenyl group, a naphthyl group, or a monocyclic or dicyclic heterocyclic group, respectively substituted by (Y).sub.n ; R.sub.4 and R.sub.5 each are a hydrogen atom, a lower alkyl group, or a cycloalkylidene group with a carbon atom combined therewith; l is 0 or an integer of 1, 2 or 3; m is 0 or 1; n is an integer of 1, 2, 3, 4 or 5; and R.sub.1, R.sub.2 and R.sub.3 are groups often used in the conventional .beta.-lactam compounds) have strong antimicrobial activities against a wide range of Gram positive and negative bacteria; particularly Gram negative bacteria including Pseudomonas aeruginosa, Pseudomonas sepatia and various Enterobacters.