We investigated the proliferation-inducing effects of human recombinant interleukin-7 (IL-7) on acute lymphoblastic leukemia (ALL) cells. It is shown that IL-7 stimulates DNA synthesis in ALL cells of B-cell precursor (n = 5) as well as immature T-cell origin (n = 2). Cytogenetic analysis of the cells of four patients proliferating in IL7- supplemented cultures established the leukemic descendence of the IL-7- responsive cells. 125I-IL-7 binding experiments with the cells of one patient and with two ALL cell lines showed the presence of two types of IL-7 receptors: one with a high affinity (kd 29 to 51 pmol/L) and one with a low affinity (kd 2.3 to 76 nmol/L) for the ligand. We conclude that IL-7 is one of the cytokines involved in the complex regulation of ALL cell proliferation.