DURING the course of a project1 on the use of penicillin in the treatment of chronic osteomyelitis, recommended by the Committee on Medical Research and contracted for by the office of Scientific Research and Development with the Hospital for Joint Diseases, the opportunity arose for the initiation and the subsequent development of several collateral studies. All these were briefly discussed in the report2 submitted at the official termination of the project, and others3 were subsequently reported with additional data in greater detail. This paper concerns itself with the use of penicillin as a precautionary measure against infection during the course of surgical procedures with primary closures of the wounds of bones and joints on (1) patients without osteomyelitic lesions, (2) patients with inactive osteomyelitic lesions, (3) patients with active infections and (4) patients with postoperative infections. These studies were closely coordinated with studies on the bacteriologic status