BACKGROUND: Src family kinases control multiple cancer cell properties including cell cycle progression, survival, and metastasis. Recent studies suggest that the Src inhibitor dasatinib blocks these critical cancer cell functions. METHODS: Because the molecular mechanism of action of dasatinib in breast cancers has not been investigated, we evaluated the effects of dasatinib as a single agent and in combination with the commonly used chemotherapeutic doxorubicin, on the proliferation, viability, and invasive capacity of breast cancer cells lines earlier categorised as dasatinib-sensitive (MDA-MB-231) and moderately resistant (MCF7 and T47D). We also tested the effects of these drugs on the actin cytoskeleton and associated signalling pathways. RESULTS: The cell lines tested varied widely in sensitivity to growth inhibition (IC(50)=0.16-12.3 microM), despite comparable Src kinase inhibition by dasatinib (IC(50)=17-37 nM). In the most sensitive cell line, MDA-MB-231, dasatinib treatment induced significant G(1) accumulation with little apoptosis, disrupted cellular morphology, blocked migration, inhibited invasion through Matrigel (P<0.01), and blocked the formation of invadopodia (P<0.001). Importantly, combination treatment with doxorubicin resulted in synergistic growth inhibition in all cell lines and blocked the migration and invasion of the highly metastatic, triple-negative MDA-MB-231 cell line. CONCLUSION: The observed synergy between dasatinib and doxorubicin warrants the re-evaluation of dasatinib as an effective agent in multi-drug regimens for the treatment of invasive breast cancers.