The effect of cyanocobalamin [c-lactam] on the incorporation of [1-¹⁴C] propionic acid 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% non-dialyzed 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. The medium was supplemented with nondialyzed serum to maintain the viability of cells grown with the inhibitor. The cells were grown for several weeks and then assayed in their culture medium. The results are the mean from three 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 (Wilcoxon) for (a) versus (b) and (b) versus (d) only.