Transforming growth factor (TGF)-β has been reported to exert growth inhibitory activity in normal epithelial cells whereas it induces cell proliferation and invasive phenotypes in advanced carcinomas. Our previous study showed that MCF10A, a spontaneously immortalized ‘normal’breast epithelial cell line, is resistant to TGF-β-induced growth inhibition, suggesting that conversion of TGF-β growth inhibitory signaling into an oncogenic pathway may occur at the early stage of tumor development/progression. To address this issue, we investigated the TGF-β signaling pathway and its role in phenotypic transformation of MCF10A cells. TGF-β treatment induced changes in the MCF10A cell morphology from cuboidal to an elongated spindle-like shape, accompanied with down-regulation of epithelial cell marker E-cadherin. TGF-β treatment was sufficient to induce migrative and invasive phenotypes in these cells, an important phenotypic conversion during tumor progression. We also showed that TGF-β treatment rapidly activated ERK-1/2 and p38 MAPK leading to upregulation of matrix metalloproteinase (MMP)-2 and MMP-9. Using chemical inhibitors and dominant negative mutants of MAPKs, we provide evidence that while both p38 MAPK and ERKs are required for TGF-β-induced MCF10A cell migration and invasion, TGF-β-induced MMP-2 and MMP-9 expression depends on p38 MAPK signaling, but is independent of ERK activity. This study demonstrates the roles of TGF-β signaling pathways for induction of oncogenic signaling in preneoplastic human breast epithelial cells and will deepen our understanding of TGF-β signaling in the progress of breast cancer.