A female patient who presented with left empyema thoracis caused by Actinomyces odontolyticus is reported. She responded to treatment with penicillin and metronidazole but after 3 weeks developed leucopenia complicated by gram-negative septicaemia. Leucopenia improved rapidly on withdrawal of metronidazole. Treatment was continued with a prolonged course of penicillin and she made an uneventful recovery.