In June 1942 the Committee on Chemotherapeutic and Other Agents of the National Research Council was requested to organize a cooperative clinical study to evaluate the effectiveness of penicillin in the treatment of bacterial infections. A relatively large group of clinics was asked to participate in the investigation in order that adequate data might be accumulated as rapidly as possible. The preliminary results of the cooperative undertaking have been summarized in a recent publication.1 The present report deals with a more detailed account of the use of penicillin2 in the treatment of 103 patients in this clinic. Our experience with penicillin in the treatment of syphilis will be published elsewhere. METHODS Selection of Patients.— In this study preference was given to patients with fulminating bacterial infections, particularly those known to be unresponsive to sulfonamide chemotherapy. Less severe infections were treated in a small group of patients known to