Three patients with T-cell lymphoma were studied with a combined cytogenetic-immunological analysis (MAC) as well as by Southern blotting analysis. The MAC method, which allows simultaneous study of both chromosomes and immunophenotype of a mitotic cell, revealed that in each case the karyotypically abnormal and, therefore, neoplastic cells originated from different stages of T-cell maturation. In one case the abnormality was seen in cells expressing CD3 and CD10, in the second in cells expressing CD3 and CD8, and in the third case in cells expressing CD4 but not CD3. Mitoses were frequent also in non-neoplastic cells, but these mitoses were normal. Southern blotting analysis revealed TCR beta rearrangements in all patients. In addition, one patient displayed TCR gamma rearrangement, and one patient a rearrangement in the IgH joining region.