An absolute methionine requirement for cell growth in culture was observed in four experimental rodent neoplasms, namely, P815/ara-C, L1210, lymphoma 5178Y, and Walker 256. Normal human fibroblast (F-136-35-56) and the human malignant cell lines HeLa and mammary adenocarcinoma (AlAb) cells in culture showed equal growth in 0.1 mM L-methionine or 0.1 to 0.4 mM DL-homocysteine. A human pancreas adenocarcinoma (Capan-1) had somewhat more stringent requirements for DL-homocysteine, whereas a human lung adenocarcinoma (A-549) responded poorly, and a human acute lymphoblastic leukemia (CCRF-HSB-2) responded not at all to equimolar or excess DL-homocysteine in the absence of L-methionine. These differences in requirement for methionine and the ability or inability to replace methionine by homocysteine indicate that a general discrimination between benign and malignant tissues on the grounds of their methionine requirement is not possible for human cells.