Disclosed are compounds of the general formula: ##STR1## wherein R.sub.1 is amino, an acylated amino or a protected amino group, X is hydrogen or methoxy, and R' is hydrogen, R or R.sup.4 where R is an organic residue attached to the azetidine ring through a carbon atom therein and R.sub.4 is azido, a halogen, an amino group which may optionally be acylated or a group of the formula ##STR2## wherein R.sub.5 is an organic residue and n is 0, 1 or 2, and pharmaceutically acceptable salts and esters thereof.The compounds have antimicrobial and/or .beta.-lactamase-inhibitory activity and are of value as drugs for human beings and domesticated animals.