The author would like to express his appreciation to Professor Ephraim Katchalski for his hospitality, to Professor Michael Sela for samples from his collection and for valuable discussion, to Professor Franz Sondheimer for the use of a spectrophotometer, and to Dr. Israel Schechter and Mr. Shmuel Shaltiel for valuable advice. Abbreviations used are: pTR, poly-L-tyrosyl ribonuclease; pApTR-L, lightly alanylated pTR, i.e., poly-DLalanyl-poly-L-tyrosyl ribonuclease; pApTR-H, heavily alanylated pTR; A(T,G)L, polv-DL-alanyl-poly(L-tyrosyl, L-glutamyl)-poly-L-lysine; poly-(L-tyrosyl, L-glutamyl)-poly-DL-alanyl-poly-L-lysine; R, ribonuclease; pAR, poly-DL-alanyl ribo-nuclease; and pTpAR, poly-Ltyrosyl-poly-DL-alanyl ribonuclease. The specific activity of I'a5 inhibitors was determined using standard solutions prepared by weighing out the solute (±0.2%). I125 activity was measured with a Packard automatic B-ray spectrometer. Enzymatic activity was assayed by the method of Hummel., Crystallographic studies: X-ray diffraction photographs were taken with a Buerger precession camera using CuKa radiation. The [hOt] and [hk0] data were collected to a resolution of approximately 4 A. Quartz calibrated photographs were used to determine the cell constants. The intensities of the reflections were determined with a Joyce-Loebl microdensitometer. E. coli K-13, a prototrophic isolate of strain K-12, and Hfr Hayes, a methionine requiring K-12 "relaxed" mutant, were used in the present experiments. Cells were grown in minimal medium buffered by 0.05 M triethanolamine at pH 8.2, supplemented with 0.5% sodium succinate and 0.2% casamino acids as sources of carbon as described previously.2 For the "step-down" experiment the enriched medium was prepared by adding 1% Difco Bacto-peptone to the minimal medium and readjusting the pH to 8.2.