Novel penicillins of the formula: ##STR1## wherein the ring A is a benzene ring or a 5 or 6-membered heteroaromatic ring containing one or two nitrogen atoms as the hetero atom, on which one or more of lower alkyl, lower alkoxy, lower alkylthio, lower haloalkyl, lower alkylenedioxy halogen, hydroxyl, nitro, free or protected amino, lower alkylamino, di(lower)alkylamino and lower alkanoylamino may be present, Z is a nitrogen atom or a methylene group, X is an oxygen atom or a sulfur atom, Y is a hydrogen atom, a lower alkoxycarbonyl group or a lower alkanoyl group, R.sub.1 is a free or protected hydroxyl group and R.sub.2 and R.sub.3 are each a hydrogen atom or a halogen atom, and their non-toxic pharmaceutically acceptable salts, and their preparation.