Corynebacterium (Rhodococcus) equi is becoming increasingly significant as a cause of bronchopneumonia and lung abscessation in foals. The organism can survive within macrophages and may thus escape normal pulmonary defence mechanisms, particularly in immunocompromised animals. The disease has hitherto been associated with mortality rates as high as 80 per cent, partly as a result of inappropriate therapy. The selection of lipid-soluble antibiotics capable of intracellular penetration is critical for the successful treatment of C equi lung abscesses. A combination of two such antibiotics, erythromycin (25 mg/kg three times daily) and rifampicin (5 mg/kg twice daily) has been used on foals since 1981. Most of these animals had radiographic evidence of extensive lung abscessation, and in all cases the presence of C equi was confirmed on culture of tracheal aspirates. The duration of therapy ranged from four to nine weeks. Mild gastritis and diarrhoea were occasionally noted, but never such as to require termination of the therapy. No other adverse side effects were encountered. The success rate, as judged by a return to normal of chest radiographs and plasma fibrinogen concentrations, has exceeded 80 per cent.