Abstract Leukopenia, granulocytopenia, and, occasionally, anemia develop in rats fed a purified diet deficient in riboflavin. Folic acid (L. casei factor) corrects the leukopenia and granulocytopenia. Riboflavin will prevent all the dyscrasias but will correct only the anemia. The bone marrow in granulocytopenic rats is hypoplastic and is almost completely depleted of cells of the granulocytic series. Cells of the erythroid series are decreased in number. The myelogram of rats made folic acid deficient by the inclusion of sulfasuxidine in a purified diet resembles this picture and in both cases the response of the marrow and the blood to folic acid therapy is similar. The bone marrow in riboflavin-deficient rats having both granulocytopenia and anemia is depleted of granulocytic cells but shows an erythroid hyperplasia. This myelogram differs from that seen in sulfasuxidine-induced folic acid deficiency anemia and granulocytopenia in which there is erythroid hypoplasia. The two anemias differ further in that the folic acid deficiency anemia responds to folic acid therapy whereas the riboflavin deficiency anemia responds to riboflavin therapy but not to folic acid therapy.