We report an acute T-lymphoblastic leukemia with a predominantly mature CD3+ CD7+ WT31+ phenotype that was induced to differentiate into different cell lineages by various recombinant human growth factors. In the presence of IL-3 or GM-CSF, the leukemic cells gave rise to myeloid and monocytic cells including terminally differentiated, partially functional, segmented neutrophilic granulocytes as assessed by morphologic, cytochemical, immunophenotypic, and functional criteria. In the presence of IL-2, leukemic granulated lymphoid cells exhibiting MHC-unrestricted cytotoxicity and expressing a CD2+ CD3+ CD5+ CD7+ CD8+ CD33+ WT31+ Leu19+ phenotype arose. Leukemic cell cultures initiated with IL-3 yielded growth factor-independent cells with a mixed lineage phenotype and morphologic and cytochemical evidence of immature blasts. These were T lymphocyte and myeloid surface antigen (CD2,CD3,CD5,CD7,CD13,CD33,WT31) positive. Identical rearrangements of the constant region of the TCR-delta gene and of the joining regions of the TCR-beta, -gamma, and -delta genes were observed in the fresh and all cultured leukemic cells, indicating that they were derived from the same malignant clone. Consistent with the molecular genetic data, the cytogenetic analyses of the GM-CSF-, IL-3-cultured and the growth factor-independent leukemic cells showed the presence of multiple, closely related abnormal clones, all of which had an interstitial deletion of part of the long arm of chromosome 6 and a complex 1;10;12 translocation. In conclusion, these data demonstrate the involvement of a multipotent leukemic precursor cell in this predominantly mature CD2+ CD3+ CD5+ CD7+ WT31+ T-ALL. This multipotent leukemic precursor may be susceptible to various growth factors and respond with ordered differentiation and maturation.