Abstract 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-β1 (HRG- β1). Results Cell viability was decreased in SK-Br-3 cells 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-β1 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-β1. 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 Her-receptor 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.