Elevated expression of Bmi-1 is associated with many cancers, including breast cancer. Here, we examined the oncogenic potential of Bmi-1 in MCF10A cells, a spontaneously immortalized, nontransformed strain of human mammary epithelial cells (HMEC). Bmi-1 overexpression alone in MCF10A cells did not result in oncogenic transformation. However, Bmi-1 co-overexpression with activated H-Ras (RasG12V) resulted in efficient transformation of MCF10A cells in vitro. Although early-passage H-Ras-expressing MCF10A cells were not transformed, late-passage H-Ras-expressing cells exhibited features of transformation in vitro. Early- and late-passage H-Ras-expressing cells also differed in levels of expression of H-Ras and Ki-67, a marker of proliferation. Subsets of early-passage H-Ras-expressing cells exhibited high Ras expression and were negative for Ki-67, whereas most late-passage H-Ras-expressing cells expressed low levels of Ras and were Ki-67 positive. Injection of late-passage H-Ras-expressing cells in severe combined immunodeficient mice formed carcinomas with leiomatous, hemangiomatous, and mast cell components; these tumors were quite distinct from those induced by late-passage cells co-overexpressing Bmi-1 and H-Ras, which formed poorly differentiated carcinomas with spindle cell features. Bmi-1 and H-Ras co-overexpression in MCF10A cells also induced features of epithelial-to-mesenchymal transition. Importantly, Bmi-1 inhibited senescence and permitted proliferation of cells expressing high levels of Ras. Examination of various growth-regulatory pathways suggested that Bmi-1 overexpression together with H-Ras promotes HMEC transformation and breast oncogenesis by deregulation of multiple growth-regulatory pathways by p16(INK4a)-independent mechanisms.