A study of seventy-one cases of acute hematogenous osteomyelitis, made duiring the past four years, has indicated that penicillin is a powerful and effective antibiotic in this disease. This finding has not only radically changed our ideas concerning the most satisfactory methods of management, but has altered our interpretation of the significance of some of the roentgenographic changes occurring during the course of the disease. When administered early and adequately, penicillin reduced mortality and morbidity, controlled the infection, minimized local destruction of the bone and resuiltant deformities, permitted spontaneous absorption of the involved bone, healing, and early return of normal or nearly normal function. Follow-up studies indicated that healing of the bone has been very satisfactory in the great majority of instances, with restoration of its architecture to normal or near normal and with considerably less osteoscierosis, sequestration, and impairment of function of the adjacent joints. The number of recurrences has been low, and limited almost entirely to those cases in which the amount of penicillin administered was too small, the duration of treatment too short, the intervals between injections too long, the beginning of treatment seven or more days after the onset of the disease, or the use of immobilization absent or inadequate.