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A group of synthetic antimicrobial agents which usually contain a 4-quinolinone ring structure. A few compounds belonging to this group have related nitrogen heterocyclic ring structures such as naphthyridine or phthalazine groups. All compounds in this group are substituted with an oxo group at the 4-position. The second-generation quinolones are also substituted with a 1-piperazinyl group at the 7-position and a fluorine moiety at the 6-position. The quinolones inhibit the A subunit of DNA gyrase and thus interfere with the ability of the enzyme to repair bacterial DNA breaks during replication. The compounds have a broad spectrum of activity against a variety of aerobic bacteria.