A 2-azetidinone derivative having a group of the formula ##STR1## wherein R.sup.1 and R.sup.2 are the same or different and each represents a hydrogen atom, alkyl, aryl or arylalkyl which may have a substituent at the 1-position and an amino group, at the 3-position, which may be acylated or protected, its salt or ester, and methods of producing the same, (1) a method comprising subjecting a 2-azetidinone derivative having a group of the formula ##STR2## wherein the symbols are as defined above at the 1-position and an amino group at the 3-position, its salt or ester to an acylation or protective-group introduction reaction, and (2) a method comprising reacting a 2-azetidinone derivative having a hydroxy group at the 1-position and an amino group, at the 3-position, which may be acylated or protected, or its salt with a compound of the formula ##STR3## wherein W is a halogen atom; other symbols are as defined above, its salt or ester. The above objective compounds are utilizable as excellent antimicrobial agents or as valuable intermediates for the synthesis of the same.