Ninety-two strains of Neisseria meningitidis (twenty-four recovered from CSF, four from blood, and sixty-four from oropharyngeal swabs) were tested for susceptibility to penicillin, ampicillin, minocycline, chloramphenicol, rifampicin, erythromycin, spiramycin, josamycin, sulphamethoxazole, and trimethoprim. Antibiotic activity (MIC) was determined by agar dilution. Among the 92 cultures, strains were in the following decreasing order: group B, undefined groups, and group C. 75% of strains recovered from carriers and 50% of those recovered from CSF were susceptible to 8 mg/l of sulphamethoxazole. No strain was susceptible to trimethoprim (MIC 8 mg/l). All strains were susceptible to penicillin (range MICs: 0.007-0.25; mean MIC: 0.06), ampicillin (0.015-0.5; 0.12), minocycline (0.03-0.05; 0.25), chloramphenicol (0.12-4; 0.5). and rifampin (0.007-0.25; 0.015). In vitro activity of macrolides was influenced by culture conditions. With 5 to 10% CO2, MICs were 2 to 4 times greater than those culture conditions. With 5 to 10% CO2, MICs were 2 to 4 times greater than those observed without CO2. Josamycin was more active than erythromycin and spiramycin.