Leukemias were induced by 7,12-dimethylbenzanthracene feeding of intact, thymectomized, or Freund's adjuvant-pretreated SJL mice. Four of six Thy-1-positive thymomas that arose in intact mice had a pseudodiploid stemline with one morphologically similar or identical marker. Banding analysis showed that the marker had arisen by the translocation of the distal part of one chromosome 15 to one X chromosome [t(X;ter 15)]. Two normal No. 15 chromosomes were also present in the same metaphase plates. These four Thy-1-positive lymphomas were thus trisomic for the distal part of chromosome 15. All 8 Thy-1-negative lymphomas, originating in the spleen or lymph nodes of thymectomized or adjuvant-pretreatment mice, had a trisomy of chromosome 12 and also a trisomy of either chromosome 3 or chromosome 18. These results further stress the importance of gene dosage effects, related to the distal part of chromosome 15, in Thy-1-positive T-cell leukemogenesis. The cytogenetic difference between the Thy-1-positive and -negative leukemias supports our hypothesis that nonrandom chromosomal changes in murine leukemias are dependent on the target cell type, rather than the inducing agent.