The effect of cyanocobalamin [c-lactam] on the incorporation of 5-[¹⁴CH₃]-tetrahydrofolate into TCA-precipitable material by intact 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% nondialyzed human serum, and 5% nondialyzed fetal bovine serum. Cyanocobalamin [c-lactam] and additional vitamin B₁₂ were used at 7.4 μmol/L. “Met” medium was the same, except that methionine at 100 μmol/L was used instead of L-homocysteine thiolactone. Nondialyzed serum was used to maintain the viability of cells grown with the inhibitor. The cells were cultured in the specified medium for several weeks until the time of assay, when they were washed and processed in the assay medium as described in the text. Results represent the mean from five experiments. (a) “Hcy” medium alone, (b) “Hcy”+c-lactam, (c) “Hcy”+additional vitamin B₁₂, (d) “Hcy”+c-lactam+additional vitamin B₁₂ , (e) “Met” medium alone, (f ) “Met”+c-lactam, (g) “Met”+additional vitamin B₁₂ , (h) “Met”+c-lactam+additional vitamin B₁₂ . P < .05 (t-test) for (a) versus (b), (b) versus (d), (e) versus (f ), (f ) versus (h), and (a) versus (e). ▪, nonmethionine-suppressible uptake of label; T, 1 SEM: n = 5.