DURING the recrystallization of pepsin by the method of Northrop1, which involves heating a solution of the enzyme to 45° C. at pH 4, followed by cooling, it was noticed that the warm solution deposited a considerable quantity of needle crystals totally different from crystalline pepsin; these were identified as tyrosine. Continued autolysis under these conditions yielded 35 mgm. of tyrosine in 24 hr. from 6 c.c. of a solution containing 1.0 gm. of dry pepsin. This represents approximately 40 per cent of the total tyrosine present. Investigation of the dialysed mother liquor by two-dimensional paper chromatography2,3 revealed the presence of at least thirty-four ninhydrin-positive substances (see diagram), many of which disappeared on acid hydrolysis, indicating that they are peptides. It is suspected that some of the spots are simple amino-acids, as indicated by their position and speed of development. Samples of the autolysing solution taken at 5-min. intervals during the first hour showed on paper chromatography with butanol acetic acid in one dimension that a very complex pattern of spots is present from the beginning, though extremely faint at first. Analysis in one dimension could not, of course, decide whether the initial pattern contains all the thirty-four spots finally detected; but it is provisionally concluded that the formation of the final fragments are not rate-determining steps.