Many bacterial infections, for example those in chest and abdomen, intracranial infections and infections in the female genital tract have anaerobic or mixed aerobic/anaerobic aetiology. Recent data indicate that anaerobes, alone or together with aerobes, are found in 15% to 30% of all positive bacteriological cultures. In 1988 at our laboratory, 284 (75%) of 379 positive cultures yielded aerobes only, 23 (6%) anaerobes only and 72 (19%) mixed aerobes and anaerobes. The initial choice of antibiotics in infections known or suspected to involve anaerobes should be based on the site of the infection, the anticipated aetiology and the severity of the infection. The place in therapy of antibiotics active against anaerobes is discussed.