Abstract: Since nonrandom chromosome changes in neoplastic cells have been proven to be good indicators of the site of gene alterations related to transformation,the authors examined the chromosomes of T-cell lymphomas induced in RF/J strain mice with methylnitrosourea (MNU) after long - term passage in syngeneic mice. All treated mice developed thymic lymphomas within 10 weeks after the injection. Chromosome of the thymus cells were examined at intervals before and during lymphoma development, as well as after they were passaged in syngeneic mice for periods up to 424 days. In preparations made directly from the thymus cells nonrandom numerical and structural alterations were found that involved the X,3,15,4,8,12,14 and 17 (Chromosomes showing alterations are listed in decreasing order of the frequency of their occurrence). In tumor cells passaged in syngeneic mice most of the above chromosomes were involved but the order was 15,14,X,1,5,6,3,11 and 12. The X,15,14,3 and 12 were aberrant in both direct preparations and in those from passaged cells, suggesting that these chromosomes carry genes (oncogenes or tumor suppressor genes) which，when altered,are particularly important in the multistep process of neoplastic transformation. MNU thus appears not only to be tissue specific but also to affect fairly specific chromosomes.