Methionine protects against the cytotoxic effect of cyanocobalamin [c-lactam] on HL60 cells. “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. Cyanocobalamin [c-lactam] (7.4 μmol/L) and additional concentrations of vitamin B₁₂ (7.4 μmol/L) had no effect on the doubling time. •, “Met” medium alone; ○, “Met” plus c-lactam; ▪, “Met” plus additional vitamin B₁₂ ; □, “Met” plus c-lactam and additional vitamin B₁₂ .