Activation of the ras oncogene is an important step in carcinogenesis. Human MCF‐10A mammary epithelial cells were transformed with a point‐mutated form of the Ha‐ras oncogene. Epidermal growth factor receptor (EGFR) phosphorylation levels were chronically elevated after EGF induction and the EGFR ligand–driven internalization rate was slower in Ha‐ras transformed MCF‐10A cells. Additionally, basal levels of p42/44 mitogen‐activated protein kinase (MAPK) expression and enzyme activity were significantly higher in Ha‐ras transformed cells, localized predominantly in the nucleus. The anti‐EGFR monoclonal antibody (MAb) 225 and the EGFR tyrosine kinase inhibitor PD153035 blocked anchorage‐independent growth of Ha‐ras transformed cells in soft agar and were more effective when used in combination. The MEK inhibitor PD98059 and anti‐erbB‐2 MAb L26 also suppressed colony formation of Ha‐ras transformed cells in soft agar. Therefore, Ha‐ras transformation leads to an augmentation in signaling through the EGFR as a result of an increase in ligand‐dependent phosphorylation, a decrease in its internalization and an up‐regulation in basal p44/42 MAPK levels. These effects may contribute to uncontrolled growth of Ha‐ras–transformed human mammary epithelial cells. Int. J. Cancer 88:44–52, 2000. © 2000 Wiley‐Liss, Inc.