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Background Human epidermal growth factor receptor-2 (Her-2) is over expressed in approximately 25- 30% of all primary breast tumors resulting in a distinctive breast cancer subtype associated with a poor prognosis and a decrease in overall survival. Trastuzumab (Herceptin®), an anti- Her-2 monoclonal antibody, has dramatically altered the prognosis of Her-2 positive breast cancer. Trastuzumab is, however, associated with primary and acquired resistance. Aim and methods To investigate the in-vitro effects of trastuzumab on cell viability (tetrazolium conversion assay), cell cycling (propidium iodide staining), apoptosis (executioner caspases and annexin- V) and relative surface Her-2 receptor expression (anti-Her-2 affibody molecule) in Her-2- positive (SK-Br-3) and oestrogen receptor positive (MCF-7) breast adenocarcinoma cells and to determine potential augmentation of these effects by two endogenous ligands, epidermal growth factor (EGF) and heregulin-ß1 (HRG- ß1). Results Cell viability was in SK-Br-3 cells decreased by exposure to trastuzumab. This was associated with G1 accumulation and decreased relative surface Her-2 receptor density, supporting the cytostatic nature of trastuzumab in vitro. SK-Br-3 cells exposed to EGF and heregulin-ß1 produced differential cell responses alone and in combination with trastuzumab, in some instances augmenting cell viability and cell cycling. Relative surface Her-2 receptor density was reduced substantially by trastuzumab, EGF and heregulin-ß1. These reductions were amplified when ligands were used in combination with trastuzumab. Conclusion Cell type specific interactions of endogenous ligands appear to be dependent on absolute Herreceptor expression and cross activation of signaling pathways. This supports the notion that receptor density of Her-family members and multiplicity of growth ligands are of mutual importance in breast cancer cell proliferation and therefore also in resistance associated with trastuzumab. [ABSTRACT FROM AUTHOR]