In 4 white infants with a macrocytic anemia characterized by a mega-loblastic type of erythropoesis in the bone marrow, a complete hematologic remission was obtained after treatment with folic acid, used either as a concentrate or in synthetic form. The response was detd. by the regular appearance of marked reticulocytosis and subsequent rise in the red blood cell count and Hb values. The bone marrow underwent an almost immediate transformation from a megaloblastic to a normoblastic pattern. It was concluded that folic acid has a specific affect on anemias of infancy with a megaloblastic bone marrow.