Human epidermal growth factor receptor2 (Her2/neu) have been found in approximately 25-30 % breast tumors and its overexpression is associated with a high risk of relapse, unfavourable prognosis and short survival.Trastuzumab has shown good clinical response in breast rumors overexpressing Her2/neu.However, only a fraction of Her2 overexpressing breast tumors respond to trastuzumab and after the initial response the resistance to trastuzumab is likely to be developed within one year.Platelet-derived growth factor receptors (PDGFR) α and β are linked with advanced human breast cancers and PDGFRs produced in car(c)nomas are generally thought to act on the nonepithelial tumor stroma by promoting angiogenesis.Still it is unclear how expression of PDGFRs contributes to the resistance to trastuzumab.We therefore have assessed interactions between epithelial breast cancer cells and human fibroblasts upon trastuzumab treatment in a co-culture model.