OBJECTIVE: To evaluate bacterial enzymes, which transform folates and anti-folates, for their activity to inhibit tumor growth and/or to improve the therapeutic index of methotrexate. APPROACH: The Walker 256-carcinosarcoma in the rat is the experimental tumor system for testing the anti-neoplastic effectiveness of purified enzymes. The evaluation of anti-folate enzymes as agents in improving the therapeutic index of antifolates is carried out with the L1210 leukemia in BDF1 mice.