Ascorbic acid does not cause cyanocobalamin [c-lactam] to develop a cytotoxic effect on HL60 cells grown in methionine. “Met” medium was RPMI 1640 without L-homocysteine thiolactone and without folic acid, containing methionine at 100 μmol/L, 5-methyltetrahydrofolate at 200 nmol/L, vitamin B₁₂ at 3.7 nmol/L, 5% dialyzed human serum, and 5% dialyzed fetal bovine serum. Ascorbic acid was used at 284 μmol/L, and cyanocobalamin [c-lactam] at 7.4 μmol/L. Cyanocobalamin [c-lactam] does not prolong the doubling time in the presence of ascorbic acid, which stabilizes 5-methyltetrahydrofolate. This suggests that spontaneous oxidation of 5-methyltetrahydrofolate to a form that can bypass methionine synthase and enter cellular folate pools directly when the functions of vitamin B₁₂ are inhibited is not the reason why cells continue to proliferate in the presence of cyanocobalamin [c-lactam] and methionine. •, “Met” medium plus ascorbate; ○, “Met” plus ascorbate and c-lactam; ▪, “Met” plus ascorbate and additional vitamin B₁₂ ; □, “Met” plus c-lactam and additional vitamin B₁₂ .