The minimal inhibitory concentrations (MICs) of twelve 4-quinolone antimicrobials and sulphadiazine were determined for 160 clinical isolates of Neisseria meningitidis. The bacteria were recovered from nasopharyngeal carriers and cases of meningitis examined in The Gambia, West Africa, during the 1982-83 dry season. MICs were determined using an agar dilution technique in Mueller-Hinton agar supplemented with 10% lysed horse blood. The inoculum used was approximately 10(4) colony-forming units of each organism, contained in 10 microliters of Mueller-Hinton broth, which was applied to the agar plates using a multipoint inoculator. Following inoculation, plates were incubated for 18 h at 37 degrees C in an atmosphere enriched to 5% carbon dioxide. The MIC of each antimicrobial for each isolate examined was determined as the lowest concentration of the antimicrobial which completely inhibited growth of the inoculum. The minimum concentrations of each antimicrobial required to inhibit 50% (MIC50) and 90% (MIC90) of the isolates examined were also determined. The more recently synthesised 4-quinolones were very active against the isolates of Neisseria meningitidis, ciprofloxacin being marginally the most active (MIC90 0.008 micrograms/ml). The activity of the 4-quinolone antimicrobials was unaffected by the MICs of sulphadiazine required by the organisms, which ranged from 0.5- greater than 64 micrograms/ml.