Anti-bacterially active penicillin 1,1-dioxides of the formula ##STR1## or a salt thereof, in which R.sub.1 denotes a hydrogen atom or an ester-forming radical,R.sub.2 denotes a hydrogen atom or optionally substituted alkoxy group,R.sub.3 denotes a hydrogen atom, COR.sub.4, SO.sub.2 -alkyl, SO.sub.2 -aryl or an optionally substituted alkyl group, but R.sub.2 and R.sub.3 do not simultaneously denote hydrogen atoms, andR.sub.4 denotes a hydrogen atom or any of many possible organic radicals.The compounds are also inhibitors of .beta.-lactamases so they can be used in conjunction with .beta.-lactamase-susceptible antibiotics. They are useful in fighting bacterial infection, in promoting animal growth and in preserving various materials.