Minimal inhibition concentrations (MICs) were determined for linezolid (LZD) and compared with those of reference antibiotics against 265 anaerobes. For the Bacteroides fragilis group, MIC range for LZD was 2-4 mg/l. Strains resistant to the other antibiotics were detected including one strain of B. fragilis showing high level resistance to metronidazole (64 mg/l). LZD MIC(50) was 4 mg/l for prevotella and 1 mg/l for fusobacteria. LZD MICs were <1 mg/l for porphyromonas and veillonellae and </=4 mg/l for all Gram-positive anaerobes. For all anaerobes, resistance rates were 1, 2.4, 9.8 and 17% for imipenem, amoxycillin-clavulanic acid, metronidazole and clindamycin, respectively. As no strains resistant to LZD were detected, this antibiotic seems a promising candidate to treat infections caused by anaerobes.