The cytotoxic effect of cyanocobalamin [c-lactam] on HL60 cells. “Hcy” medium was RPMI 1640 without methionine and without folic acid, containing L-homocysteine thiolactone at 200 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] and additional vitamin B₁₂ were used at 7.4 μmol/L. Cyanocobalamin [c-lactam] caused growth arrest followed by cell death, and this effect was reversed by additional concentrations of vitamin B₁₂ . In nondialyzed serum, cell death did not occur (data not shown). High concentrations of 5-methyltetrahydrofolate (up to 20 μmol/L), did not protect the cells against the inhibitor (data not shown). •, “Hcy” medium alone; ○, “Hcy” plus c-lactam; ▪, “Hcy” plus additional vitamin B₁₂ ; □, “Hcy” plus c-lactam and additional vitamin B₁₂ .