Thiouracil and propylthiouracil fed to rats on purified diets produced severe granulocytopenia, and general leukopenia.

These blood dyscrasias were prevented by simultaneous administration of pteroylglutamic acid or liver in amounts to furnish at least 50 γ per cent of folic acid activity.

The leukopenia and granulocytopenia due to thiouracil could not be altered by excessive administration of pyridoxine hydrochloride.

The anemia in rats due to propylthiouracil was not appreciably influenced by folic acid.

ACKNOWLEDGMENTS We wish to acknowledge the assistance of Alice M. Bergdahl and E. M. Stapert in obtaining the hematologic data.