Antibacterial activity is exhibited by 2-azetidinones having an acylamino substituent in the 3-position and having an activating group in the 1-position of the formula
wherein R4 is -CH2-Z,
or
and Z is
Antibacterial activity is exhibited by 2-azetidinones having an acylamino substituent in the 3-position and having an activating group in the 1-position of the formula ##STR1##