In a double blind trial erythromycin was compared with a combination of ampicillin and amoxycillin for treating adults admitted to hospital with primary pneumonia. The clinical course of 42 patients treated with ampicillin and amoxycillin was similar to that of the 49 in the erythromycin group. Fall in temperature, symptomatic recovery and radiographic improvement were similar (two-thirds made an uncomplicated recovery). Infusion-related phlebitis was more common with erythromycin. Otherwise adverse reactions were unusual. The outcome was related principally to the cause of the pneumonia with bacteraemic/antigenaemic pneumococcal pneumonia, Legionnaires' disease, other bacterial pneumonias and psittacosis having a poor prognosis. Both forms of antibiotic therapy gave similar results but we suggest that a combination of erythromycin with ampicillin may be logical initial treatment for severe pneumonia of unknown cause.