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100; on a CDr0 unitage basis, 290 to 100. The relative efficacy of the various penicillins was closely comparable with that previously reported by ourselves 1We are indebted to Dr. R. Pasteruack and Dr. E. V. Brown of the Department of Research Chemistry of Chas. Pfizer & Co. for the preparation of the crystalline or purified penicillins dihydro-F, G, and K used in this study and to Mr. E. J. Goett of the Department of Research and Development, Chas. Pfizer & Co., for preparation of the impure fractions. We are further indebted to the Lederle Laboratories, Inc., for the preparation of crystalline penicillin X. Analyses of the impure fractions were carried out by Mr. T. Grenfell of the Analytical Department of Chas. Pfizer & Co.