The effects of various concentrations of methionine on the growth inhibitory effects of cyanocobalamin [c-lactam] on HL60 cells. “Hcy” culture 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, to which was added methionine to give the concentrations shown. Cyanocobalamin [c-lactam] was used at 7.4 μmol/L. HL60 cells were grown for 4 weeks under these conditions, and cell counts were performed every 48 hours. The doubling time was calculated from the slope of the regression line between the log of the cell count and the duration of culture. The proliferation rate was calculated as the number of doubling times per 24 hours. Because small concentrations of methionine, equivalent to those present in nondialyzed serum, protect the cells against the inhibitor, its cytotoxicity is caused by methionine deficiency.