This invention relates to new 2-azetidinone compounds, which have antimicrobial activities, and to processes for the preparation thereof, and more particularly, this invention provides new 2-azetidinone compounds, especially ones having various substituted carboxyalkyl radicals at the first position and having various group at the fourth position of the azetidinone nucleus, which have antimicrobial activity against various pathogenic microorganisms and are useful as antibiotics in treatment for microbial infections in mammals including human beings and animals.