Primary cultures and established cell lines derived from human T-cell leukemias were analyzed for genomic rearrangements in the region 3' of the MYC locus. A T-cell leukemia line, HUT 78, whose 3' MYC region is rearranged, carries a chromosome t(2;8) juxtaposition; i.e., a locus derived from chromosome region 2q34 is attached to the 3' end of one MYC allele. The t(2;8) rearrangement in the HUT 78 cell line results in expression of a fused transcript encompassing the MYC gene and a locus designated TCL4 (T-cell leukemia/lymphoma 4), which normally resides on chromosome 2. The steady-state level of MYC-TCL4 fusion transcripts in HUT 78 cells is significantly higher than the MYC RNA level found in several other B- and T-cell lines. The production of fused MYC-TCL4 transcripts in a leukemic cell line raises the possibility that other B- and T-cell leukemias may express MYC fusion transcripts as an integral step in their pathogenesis.