Development of breast cancer has been associated with deletions at multiple chromosomal regions, including 6q, 11p, and 11q. In this study we analyzed the effects of the introduction of chromosomes 6 and 11 on the cell phenotype of the breast cancer cell lines MDA-MB-231 and MCF-7. Chromosome 6 induced alterations of in vitro growth properties and suppressed tumorigenicity of MDA-MB-231 cells. Spontaneous reduction of the transferred chromosome allowed mapping of the tumor suppressor gene(s) to region 6q21-q23 and/or 6q26-q27. Clones MCF-7/H6 underwent a senescence process that lasted five months. Chromosome 11 had no effect on MDA-MB-231 cells, although it suppressed tumorigenicity of MCF-7 cells. A MCF-7/H11 clone lacking the short arm of the transferred chromosome retained tumorigenicity, however, tumor cell growth was significantly reduced. These results suggest that each chromosomal arm may contain genes important for the suppression of MCF-7 tumorigenic properties.