A levo isoform of leucovorin calcium with antineoplastic activity. Levoleucovorin is an active metabolite of folic acid, which does not require metabolism by dihydrofolate reductase. This agent counteracts the toxic effects of other folic acid derivative agents, rescuing the patient while permitting the antitumor activity of the folate antagonist. This agent also potentiates the effects of fluorouracil and its derivatives by stabilizing the binding of the drug's metabolite to its target enzyme, thus prolonging drug activity.