Metabolic requirements of malignant cell lines derived from patients with chronic myelogenous and acute lymphoblastic leukemias were compared to those of proliferating normal cells (mitogen-stimulated human lymphocytes) and circulating blasts from acute myeloblastic and acute lymphoblastic leukemias. Requirements were judged by degree of amino acid (AA) utilization in short-term cultures and assessed by the effect of selective AA deprivation on cell growth. Cell growth was measured by DNA synthesis and growth rate analysis. Six AAs (serine, threonine, methionine, valine, phenylalanine, and lysine) were appreciably utilized (52-87%) by IM-9, CEM, MOLT-4, and K-562 cells, but little or no utilization of these or any other AAs were noted in HSB cells, in leukemic blasts, or in mitogen-stimulated normal lymphocytes in short-term culture. Omission of lysine from culture media greatly inhibited cell growth (DNA synthesis by 91%), and cell density (by 83%) of IM-9 cells. However, omission of lysine, valine, serine, threonine, methionine, or phenylalanine had less of an effect on CEM and MOLT-4 cell lines. These observations demonstrate that under the conditions used the IM-9 cell line is uniquely dependent on extracellular lysine levels in contrast to the other cell lines studied. This suggests that human malignancies other than acute lymphoblastic leukemia which exhibits an obligate dependence on extracellular asparagine might be manageable by enzymatic degradation in vivo or by dietary restriction of indispensable AAs.