A human leukemia cell line, PER-255, was established from the bone marrow of a 5-year-old boy with features typical of lymphomatous T- acute lymphoblastic leukemia (T-ALL). The leukemic origin of cell line PER-255 is indicated by its cytochemical and immunologic similarity to the patient's fresh leukemic cells, which correspond to immature cortical thymocytes. Southern blot analysis showed that the IgJH genes were in germline configuration, whereas both alleles of the T-cell receptor-beta (TCR-beta) gene were rearranged in PER-255 cells, with identical rearrangements present in the patient's leukemic cells. Cytogenetic analysis of the cell line revealed a single abnormal clone with the karyotype 46,XY,t(7;10)(q32–34;q24),t(9;12) (p22;p12–13). Reciprocal translocations involving chromosome bands 7q32–36, containing the gene for the TCR-beta chain, have been reported for a number of tumors of T-cell origin. Translocations involving the 7q32–36 region appear to be nonrandomly associated with childhood T-ALL, whereas abnormalities of 9p and 12p have been reported to be nonrandomly involved in ALL but not specifically associated with the T- cell phenotype.