The uptake of tritiated thymidine by leukemic and normal spleens has been studied. The values obtained for the normal spleen were always higher than those for the leukemic spleen. When virus-induced leukemic spleens were cultured in high concentrations of folic acid their uptake equalled that of normal spleen. This was not true of a non-viral leukemia or of a lymphosarcoma. When normal splenic cultures were infected with the Friend leukemia virus, the number of labeled cells decreased; but when high concentrations of folic acid were added, the number increased.