Some malignant and transformed cell lines are unable to proliferate in vitro in a L-methionine-depleted medium supplemented with L-homocysteine. To investigate the utilization of preformed and endogenously synthesized methionine 4 cell lines have been chosen with a range of abilities to proliferate under such nutritional conditions. The order of the ability of these cell lines to proliferate in an L-methionine-depleted medium containing 0.1 mM L-homocysteine parallels the minimal concentration of L-methionine required for optimal growth; L-methionine auxotrophs having a greater minimal requirement. In the presence of 0.1 mM L-homocysteine all of the cell lines synthesize macromolecules from [5-14C]methyltetrahydrofolic acid during a 24 h period, and the cell line with the highest methionine requirement shows the most extensive incorporation of radiolabel into DNA and RNA, both in depleted medium and in medium containing 6.7 microM L-methionine. Double-label experiments using [5-14C]methyltetrahydrofolic acid and L-(methyl-3H) methionine show preferential incorporation of preformed over endogenously synthesized methionine by methionine auxotrophs. There is no alteration in the intracellular level of S-adenosyl-L-homocysteine (SAH) or SAH hydrolase activity in cells incubated for 24 h in methionine-depleted medium supplemented with 0.1 mM L-homocysteine. These results suggest that certain cell lines are unable to effectively use endogenously synthesized methionine.