Forty strains of Bacteroides sp. were tested against three nitroimidazole compounds, metronidazole, tinidazole and nimorazole and the aminoglycosidic antibiotic spectinomycin. The effect of altering the inoculum and the presence of serum upon the minimum inhibitory concentration (MIC) was also noted. The three nitroimidazole compounds were all very active (average MIC = circa 0.25 mg/l. Tinidazole was twofold more active. The MIC of spectinomycin was 32 mg/l