Penicillins of the formula: ##STR1## wherein R is an unsubstituted phenyl group or a substituted phenyl group having one or more hydroxyl groups or a cyclohexadienyl group and A represents a monocyclic or polycyclic nitrogen-containing heterocyclic aromatic ring, which are useful as antimicrobial agents having a broad antimicrobial spectrum against gram-positive and gram-negative bacteria including Pseudomonas and can be produced advantageously from the phenacyl ester of 6-aminopenicillanic acid through a series of steps.