5-Formyl-tetrahydrofolate [citrovorum factor (CF)] is commonly used for preventing or reversing toxicity due to treatment with high-dose methotrexate (HDMTX). In vivo, CF is converted to 5-methyltetrahydrofolate (MTHF) and then to the 5,10-methylene tetrahydrofolate, which is the active coenzyme involved in thymidine synthesis. In this study, MTHF was used for protection from toxicity following larger doses of MTX than previously studied, doses which could not be tolerated without effective "rescue." Fifteen patients were given 18 courses of MTX in escalating doses (1-24 g) followed by MTHF. The toxic effects observed included: leukopenia, thrombocytopenia, mucositis, nausea, vomiting, and elevation of serum creatinine and serum transaminase. The side effects were reversible and all patients recovered fully. "Matched pairs" comparison of CF and MTHF rescue in five patients showed no difference in rescue efficacy between the two agents. Since ten of the treatments consisted of 12 and 24 g of MTX, doses potentially fatal without rescue, MTHF is an effective agent for prevention of MTX toxicity.