1. Rhesus monkeys fed purified rations supplemented with adequate amounts of the B vitamins, ascorbic acid, and whole liver substance maintained the following average blood picture:

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2. Natural diets or purified rations supplemented with liver extract do not support the above blood picture. The hemoglobin is lower and there is an increase in the range of the total leukocyte count and in the neutrophil-lymphocyte ratio to 2.0 ([unknown]). These figures are similar to the values in the literature and generally accepted as the normal.

3. Previous reports have shown the characteristic blood dyscrasias which develop when monkeys are fed certain B vitamin-deficient diets. These changes are summarized graphically in this paper.

4. The importance of determining the concentration of hemoglobin and the formed elements of the blood as a diagnostic test in nutritional studies has been shown.

We wish to acknowledge our indebtedness to Merck and Co., Rahway, N. J., for some of the crystalline vitamins; to Wilson Laboratories, Chicago, Ill., for the various liver preparations; and to Lederle Laboratories, Inc., Pearl River, N. Y., for synthetic folic acid. The authors are grateful to Miss Ethel Thewlis for aid in determining cellular elements and to Drs. Harry A. Waisman and James H. Shaw for assisting in early parts of the work.