The active l-isomer of the racemic mixture of the 5-formyl derivative of tetrahydrofolic acid. Metabolically active, l-leucovorin, also known levoleucovorin, does not require bioactivation by dihydrofolate reductase, an enzyme inhibited by folic acid antagonists. This agent may enhance the effects of fluoropyrimidines by stabilizing their binding to the enzyme thymidylate synthase.