A NUMBER of otolaryngologic cases in which penicillin was injected intramuscularly and applied topically were followed by me over a period of one year, from February 1944 to February 1945.Much has been written recently in the literature about penicillin, but little has been said regarding the topical application of the agent in otolaryngologic cases or of the bacteriologic aspects of these cases in relation to the clinical work. Penicillin has been found to be effective in the presence of pus, peptones, tissue autolysates and other substances that are inhibitory of sulfonamide compounds.1Penicillin is essentially nontoxic, and the toxic reactions attributed to it were apparently due to toxic impurities rather than to penicillin.2 Lierle and Evers3 have shown that penicillin has practically no irritating effect on the cilia of the nasal mucosa, even when the concentration of penicillin is as high as 5,000 Oxford units per cubic centimeter of isotonic solution of sodium chloride, which is a much higher concentration than any used in this series of cases.On the basis of the results in a case of osteitis of the left antrum, which was the first case in which pure penicillin was used locally in the nose at the Massachusetts General Hospital, it appeared that penicillin distributed throughout the nose would be a good thing. It occurred to me that penicillin should be effective in the respiratory tract because of the anatomic structure and the accessibility of its surfaces for local application. I decided, therefore, to make studies on the antibacterial and the local action of penicillin, the first of which was on penicillin as an antibacterial agent. Fifty cases in which penicillin was administered were selected and followed.