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 groups at the fourth position of the azetidinone nucleus, which have antimicrobial activities against various pathogenic microorganisms and are useful as antibiotics in treatment for microbial infections in mammal including human being and animals.