Levomefolate-13C-d3 is intended for use as an internal standard for the quantification of levomefolate by GC- or LC-MS. Levomefolate is the single (S) isomer of 5-methyltetrahydrofolic acid and an endogenous active form of folic acid . It donates a methyl group to homocysteine in the biosynthesis of methionine. Levomefolate (2 mg/kg) prevents decreases in circulating polymorphonucleates induced by methotrexate in an L1210 murine skin lymphocytic leukemia model, and it increases survival in the same model when administered in combination with methotrexate. Formulations containing levomefolate have been used as dietary supplements, as well as in oral contraceptives for the prevention of neural tube defects in unintended pregnancies.