The quinolones are new antimicrobial agents that are descendants of nalidixic acid. As a new class of synthetic antimicrobials they exhibit a wide spectrum of activity. Newer quinolones such as ciprofloxacin, norfloxacin, enoxacin, and pefloxacin are highly active against gram negative bacilli of the Enterobacteriaceae, Neisseria gonorrhoeae, Haemophilus influenzae and Haemophilus ducreyi. They exhibit less activity against Pseudomonas sp. and Staphylococcus sp., and little in vitro activity against Streptococcus pneumoniae or Enterococci. Their activity against Mycobacterium sp. is variable, and obligate anaerobes are usually considered resistant. Of the quinolones reviewed, ciprofloxacin displays the highest degree of in vitro activity.