Methyl folic acid,2 N-(4-(4-quinazoline) amino) benzoyl)-glutamic acid, the Mg salt of formyl pteroyl glutamic acid, the Mg salt of formyl pteroic acid, pteroyl aspartic acid, oxyfolic acid and oxypteroic acid have been studied as to their effect on blood regeneration in selected cases of Addisonian pernicious anemia, nutritional macrocytic anemia and tropical sprue. In the amounts administered, only the Mg salt of formyl pteroyl glutamic acid was effective in producing reticulocytosis and an increase in red blood cells, hemoglobin, white blood cells and platelets, and it was not as effective per unit of weight as was folic acid per se. Presumably this compound is slowly changed into folic acid in the body. It is of special interest that the Mg salt of formyl pteroic acid (Streptococcus lactis factor) was negative in producing hemopoiesis. These observations show the very great specificity of the folic acid molecule.