Molt-3 and Molt-4 are T-cell lines originally derived in 1971 from a patient with T-cell acute lymphoblastic leukemia. An unusual T-cell antigen receptor gamma-chain gene (T-gamma) rearrangement detected by Southern blot analysis of Molt-4 prompted an in-depth study of the immunophenotype and karyotype of both cell lines. Molt-3 and Molt-4 had immunophenotypic characteristics of thymocytes with expression of CD1 and CD5. Both cell lines had a hypertetraploid karyotype with two rearranged no. 7 chromosomes: 2der(7)t(7;7)(p15;q11). The presence of a break in chromosome band 7p15 suggested the involvement of T gamma. We cloned the rearranged BamHI fragments spanning the known T-gamma constant and joining regions. Comparison with germline clones of T gamma did not suggest any of the clones included a breakpoint region. Thus the 7p15 chromosomal abnormality in Molt-3 and Molt-4 is not associated with the currently described joining and constant regions of T gamma.