MCF-10A cells are a spontaneously immortalized untransformed human mammary epithelial cell line. We have previously shown that overexpression of a human point-mutated c-Ha-ras proto-oncogene, the rat c-neu (c-erbB-2) proto-oncogene, or the human transforming growth factor-α (TGF-α) gene in MCF-10A cells leads to in vitro transformation of such cells. To ascertain whether the introduction of two of these genes into MCF-10A human mammary epithelial cells induces a completely tumorigenic phenotype, we infected MCF-10A Ha-ras and MCF-10A TGF-α cells with a recombinant retroviral vector containing the human c-erbB-2 proto-oncogene and the hygromycin-resistance gene. Ten MCF-10A TGF-α/c-ethB-2 (MCF-10A TE) and 10 MCF-10A Ha-ras/c-erbB-2 (MCF-10A HE) hygromycin-resistant clones were randomly selected and expanded into cell lines. MCF-10A TE and MCF-10A HE clones expressed a 10-fold to 40-fold increase in p185 erbB-2 protein levels compared with parental uninfected cells. These cells exhibited a fourfold increase in their growth rate in serum-free medium and showed a strongly reduced mitogenic response to exogenous epidermal growth factor or TGF-α compared with MCF-10A cells. Moreover, both MCF-10A TE and MCF-10A HE clones exhibited a fivefold to 20-fold higher cloning efficiency in soft agar than MCF-10A Ha-ras, MCF-10A c-erbB-2, or MCF-10A TGF-α clones. However, neither MCF-10A TE nor MCF-10A HE cells were able to grow as tumors in vivo when they were injected into nude mice. These results suggest that c-Ha-ras, c-erbB-2, and TGF-α genes have an additive effect on the in vitro transformation of an immortalized human mammary epithelial cell line, but that additional genetic changes such as activation of other proto-oncogenes or inactivation of a tumor suppressor gene may be necessary to elicit a fully tumorigenic phenotype. © 1992 Wiley-Liss, Inc.