Trisomy 15 is the most common chromosomal aberration in murine T-cell lymphomas. The relevant chromosomal region responsible for the growth advantage of the 15-trisomic cell has not been defined. In order to map this region, we have induced thymic lymphomas by chemical carcinogens (DMBA or MNU) in mice with 2 different constitutional translocations, T(7;15)9H homozygotes and [T(7;15)9H x T(5;15)4Ad] F1 hybrids. Twenty-two tumors developed in 90 carcinogen-treated mice. Among the 14 cytogenetically analyzed thymic lymphomas, 4 were diploid and 5 were aneu-ploid, with no chromosome-15-associated changes. Five lymphomas showed partial duplication of chromosome 15. Four of them have duplicated the segment distal to the C/DI breakpoint of T9H, while the 5th carried an interstitial duplication of the D2 sub-band of the T(7;15) translocation chromosome. These findings suggest that the duplication of the D 2/3 region, known to contain the c-myc and the pvt-1 genes (Banerjee et al., 1985), rather than other regions of chromosome 15, contributes to the development and/or progression of murine T-cell leukemias.