Considerable controversy exists as to whether or not antibiotics should be administered "prophylactically" to patients with penetrating chest trauma. No prospective study of this problem has been reported. Therefore, 75 patients with isolated, penetrating chest injury were randomized prospectively in a double-blind study. Group A patients (38 patients) were given 300 mg. of clindamycin phosphate every 6 hours, beginning with admission and lasting until 1 day following chest tube removal or for 5 days, whichever was shorter. Group B patients (37 patients) were given a placebo on the same schedule. The patients' hospital course, fever, white blood count, culture data, and roentgenograms were recorded serially. Clindamycin-treated patients had a significantly lower incidence of radiographic pneumonia, less fever, and a lower incidence of positive pleural and wound cultures. They acquired empyema less frequently, required fewer operations, and had a shorter period of hospitalization. Antibiotics may be useful, therefore, as adjunctive therapy in the management of penetrating chest trauma.