1. Different sulfonamides were tested to ascertain their effect in producing the characteristic symptoms of acute PGA deficiency in rats fed on synthetic diets. Sulfasuxidine (1 per cent) and the less soluble phthalylsulfathiazole (1 per cent) were equally effective. Sulfathiazole in 1 per cent concentration produced a hemolytic anemia not reversible by PGA or whole liver powder. In a 0.5 per cent concentration it was also effective, but in view of its toxicity, the less soluble sulfonamides were to be preferred. A mixture of 0.5 per cent sulfathiazole and 0.5 per cent sulfadiazine was extremely toxic and produced a hemolytic anemia. Sulfaguanidine was toxic at 1 per cent concentration.
2. When intermittent small doses of PGA were given to PGA-deficient rats to prolong their life from 45 up to 155 days, 1 per cent sulfasuxidine or phthalylsulfathiazole, or 0.5 per cent sulfathiazole were equally efficient in producing regularly a macrocytic normochromic anemia.
3. The response of PGA-deficient rats to single doses of PGA has been studied and an assay procedure has been suggested which uses the weight increase and duration of cure as the measure of the response. The W.B.C. and reticulocyte response can also be used as a qualitative indication of PGA activity.
4. Of the substances tested by this procedure, vitamin B12, purified pernicious anemia preparations, ascorbic acid and xanthopterin showed no PGA activity. A commercial yeast preparation and Teropterin were found to possess biologic activity comparable with that found by other workers in assays on chicks.
ACKNOWLEDGMENTS We wish to thank Dr. E. Lester-Smith, of the Glaxo Laboratories, for the generous supply of vitamin B12, Dr. T. H. Jukes, of the Lederle Laboratories, for the gift of PGA and Teropterin, and Dr. W. Jacobson for the samples of purified P. A. liver preparations and xanthopterin.