We established a novel T cell line, designated TK-6, from a patient with T cell lineage blast crisis of chronic myelogenous leukemia (CML) complicated by hypercalcemia. A surface marker study showed T cell phenotype, cluster designation (CD)4, CD5 and CD7. Light and electron microscopic examination revealed myeloperoxidase (MPO)-negative, however, ultrastructural examination under certain specific conditions demonstrated that some cells were MPO-positive. The TK-6 cell karyotype carried a t(9;22)(q34;q11) and additional chromosome aberrations, including a deletion of the long arm of chromosome 6 and the abnormality of chromosome 7. Southern blot analysis showed rearrangement of the T cell receptor beta-chain (TCR beta) gene and the major breakpoint cluster region (bcr) gene. Northern blot analysis detected the expression of the parathyroid hormone-related protein (PTHrP) gene, however, the proviral genome of human T cell leukemia virus type I (HTLV-I) was negative. This cell line will provide a valuable resource for the analysis of the relationship between T cell lineage crisis and myeloid differentiation and for the analysis of humoral hypercalcemia of malignancy (HHM) or leukemia.