This article is only available in the PDF format. Download the PDF to view the article, as well as its associated figures and tables. To the Editor: — In the September issue of the Archives of Dermatology and Syphilology there is a short note by Dr. Schoch on the local use of penicillin. I have not used penicillin in the fashion Dr. Schoch has, and, while I have no doubt of the accuracy of his observations, I should like to sound a note of caution which I hope ultimately will prove not to be needed. In a paper just read before the American Public Health Association, Dr. Welch and I have shown that repeated intradermal injections of penicillin at five day intervals sensitized 9 out of 9 persons. The majority of the reactions were of the Arthus type. While none of the reactions were severe, this may have been because when the reactions became manifest we did not continue with further injections. What effect such a sensitization would have on the therapeutic use