Overexpression of the B cell leukemia/lymphoma-2 (bcl-2) gene has been shown to confer a survival advantage on cells by inhibiting apoptosis. In epithelia, the bcl-2 gene is also related to development and differentiation, and the protein is strongly expressed in the embryo in the epithelial cells of the developing mammary gland. To investigate directly the effect of bcl-2 on human epithelial cells, we used an amphotropic recombinant retrovirus to introduce the gene into nontumorigenic cell lines developed from luminal epithelial cells cultured from milk. Here we demonstrate that while bcl-2 overexpression does not directly induce the tumorigenic phenotype, it provides a survival advantage to the mammary epithelial cells by inhibiting cell death at confluence or under conditions of serum starvation, bcl-2 can also affect the phenotype of the original epithelial cells, and promote epithelial-mesenchymal conversion, accompanied by loss of the cell adhesion molecules E-cadherin and alpha 2 beta 1 integrin. The extent of the epithelial-mesenchymal conversion varies with small differences in the phenotype of the parental line and with the level of expression of Bcl-2 and in some cases cell lines emerge with a mixed phenotype. The increased survival of Bcl-2-expressing cells at confluence results in multilayering, and the development of three- dimensional structures. Where a mixed phenotype is observed these structures consist of an outer layer of polarized epithelial cells separated by a basement membrane-like layer from an inner mass of fibroblastoid cells. Branching morphogenesis of bcl-2 transfectants is also observed in collagen gels (in the absence of fibroblast growth factors). The results strongly indicate that by increasing their survival under restrictive growth conditions, and by modifying the epithelial phenotype, bcl-2 can influence the specific morphogenetic behavior of mammary epithelial cells.