In C3H/10T1/2 murine fibroblasts, overexpression of both c-Src and the human epidermal growth factor (EGF) receptor 1 (HER1) is required for detection of stable complexes between the two molecules and results in hyperactivation of the receptor and synergistic increases in tumor formation in nude mice, as compared with cells that overexpress only one of the pair. Elevated levels or activities of c-Src and HER1 also occur in a subset of later-stage breast cancers, suggesting that interactions between these two molecules could contribute to a more aggressive clinical course. To determine whether stable complexes between c-Src and HER1 occur in human breast cancers under the same conditions as in murine fibroblasts and whether the appearance of such complexes correlates with enhanced signaling through the EGF receptor and increased tumor growth, human breast tumor cell lines and tumor tissues were analyzed for a number of c-Src/HER1-mediated signaling events and tumorigenicity. In a panel of 14 cell lines, 10 overexpressed c-Src, and of these, five contained elevated levels of HER1 and exhibited an EGF-dependent association between HER1 and c-Src. This association was also present in a HER1/c-Src-overexpressing tumor sample from a breast cancer patient. Further analysis of signaling events revealed that phosphorylation of the HER1 substrate, Shc, and its downstream effector, mitogenactivated protein kinase, was increased in EGF-stimulated MDA-MB-468, MDA-MB-231, and BT-549 cells (which overexpress both c-Src and HER1) as compared with MCF7 and ZR-75-1 cells (which only overexpress c-Src). Furthermore, MDA-MB-468 and MDA-MB-231 cells displayed increased tumorigenicity in nude mice. These results support the hypothesis that c-Src/HER1 interactions contribute to tumor progression in certain latestage breast tumor cells.