During 1989 and 1990 strains of Neisseria gonorrhoeae with reduced susceptibility to ciprofloxacin were isolated in laboratories across the United Kingdom. Treatment failures were associated with some of these infections. These strains were detected by quantitative susceptibility testing because the zone of inhibition around 5 micrograms ciprofloxacin discs shows little decrease in size even with those that are the most resistant. This study determined that strains with reduced susceptibility to ciprofloxacin (MIC of greater than or equal to 0.05 mg/l) produced no zone of inhibition around a commercially available disc containing 30 micrograms of nalidixic acid. Ciprofloxacin sensitive (MIC of less than 0.05 mg/l) strains, however, grew with a large zone (greater than 21 mm) around this disc. These observations suggest that laboratories could adopt this disc test to detect those strains for which ciprofloxacin is not appropriate treatment.