1. In agreement with findings by other workers, rats in acute pteroylglutamic acid deficiency showed leukopenia and growth depression followed by death, without any significant change in the red cell picture.
2. In chronic deficiency, however, produced by the addition of small pteroylglutamic acid doses given intermittently, a severe anemia was obtained after seventy days.
3. The anemia was macrocytic and "normochromic." Price-Jones curves showed a preponderance of macrocytes with anisocytosis. This agreed with findings by other workers for other species.
4. The anemia could be cured by single doses of 40 µg. or more of pteroylglutamic acid.
5. There was no significant difference between sexes to pteroylglutamic acid deficiency. Reduction in the protein content of the diets, containing 1 per cent sulfasuxidine, from 18 per cent to 10.5 per cent, produced no significant difference in the time of onset and severity of the blood symptoms.
6. These results were not due to infection with Bartonella muris. This infection produced a macrocytic anemia of a different type, and was curable by treatment with neoarsphenamine.
ACKNOWLEDGMENTS We are grateful to Dr. T. H. Jukes of the Lederle Laboratories for generous supplies of aldehyde-free PGA; and to Dr. K. Folkers of Merck Laboratories for the biotin used in these experiments. We wish to thank Dr. W. Jacobson for his advice during the course of this investigation. Valuable technical help was provided by Mr. D. R. Ashby, Mr. S. G. Impey, Miss M. J. Kemp and Mr. P. W. Wilson, to whom the authors are indebted.