Antimicrobial quinolonyl lactam esters comprising a lactam-containing moiety linked, by an ester group, to the 3-carboxy group of a quinolone moiety. These compounds are of the formula: ##STR1## wherein (1) R.sup.3, R.sup.4, and R.sup.5, together with bonds "a" and "b", form certain lactam-containing moieties similar to those known in the art to have antimicrobial activity; and(2) A, R.sup.6, R.sup.7, and R.sup.8 form any of a variety of quinolone or napthyridine structures similar to those known in the art to have antimicrobial activity.