This article is only available in the PDF format. Download the PDF to view the article, as well as its associated figures and tables. Penicillin is the most effective and least toxic antibiotic in general use today. As with all new chemotherapeutic agents, the initial widespread use of penicillin will become more completely defined as its field of usefulness becomes more fully understood. The value of penicillin in the treatment of acute medical and surgical infections is well established. Only by clinical investigation can its value be determined in the treatment of chronic diseases. Chronicity in pulmonary infection introduces factors not present during the acute stage. The presence of complicating mechanical factors such as tissue destruction, fibrosis, avascularity and bronchial occlusion all influence the efficacy of penicillin therapy. Furthermore, chronic pulmonary infections have a varied bacterial flora, and some of the organisms may not be penicillin sensitive. In contrast to the unparalleled value of penicillin in the treatment of acute pulmonary infections, it is of more limited value in the treatment of chronic pulmonary