We investigated the effects of interleukin-7 (IL-7), a stromal cell derived cytokine known to stimulate proliferation of murine lymphoid precursor cells, alone and in combination with IL-3, IL-1, and IL-6 on proliferation of purified blast cells in acute lymphoblastic leukemia (ALL). After 7 days of liquid culture DNA-synthesis was induced in six of 10 cALL, three of five B-ALL, and two of seven T-ALL samples by IL-7 or IL-3 or both. Monitoring of leukemic cell populations in suspension culture by Southern blot analysis of immunoglobulin and T cell receptor gene rearrangements revealed preferential stimulation of the leukemic cell clone by both IL-7 or IL-3 in one cALL and one B-ALL sample. In these cases the combination of IL-7, IL-3, and IL-1 was as effective in stimulation of DNA-synthesis as the most potent cytokine alone. There was no evidence of lymphoid maturation during liquid culture as defined by immunophenotyping using flow cytometry. Stimulation of nonleukemic cell population seen in two other cases of cALL was associated with residual erythroid and granulocyte-macrophage colony forming cells after liquid culture as defined in parallel clonogenic assays in one and detection of CD 33+ and CD 13+ cells after culture in the other cALL sample. We conclude that IL-7 directly stimulates monoclonal growth of leukemic cells in a subset of ALL without evidence of concurrent maturation induction.