Abstract. Objectives: The potential of epidermal growth factor receptor (EGFR)- and Her2-targeted antibodies Cetuximab, Pertuzumab and Trastuzumab, used in combination to inhibit cell proliferation of breast cancer cells in vitro, has not been extensively investigated. It is anticipated that there would be differences between specific erbB receptor co-expression profiles that would affect tumour cell growth. Materials and methods: We have examined the effects of Cetuximab, Pertuzumab and Trastuzumab, applied separately or in combination, on cell proliferation of BT474 and SK-BR-3 breast cancer cell lines. Cell cycle progression of BT474 and SK-BR-3 cells was statically and dynamically assessed using flow cytometry. In order to discover a potential influence of differential EGFR co-expression on sensitivity to antibody treatment, EGFR was down-regulated by siRNA in SK-BR-3. An annexinV/propidium iodide assay was used to identify potential induction of apoptosis. Results: Treatment with Pertuzumab and Trastuzumab, both targeted to Her2, resulted in a reduced fraction of proliferating cells, prolongation of G1 phase and a great increase in quiescent BT474 cells. Cetuximab had no additional contribution to the effect of either Pertuzumab or Trastuzumab when administered simultaneously. Treatment with the antibodies did not induce an appreciable amount of apoptosis in either BT474 or SK-BR-3 cells. In contrast to SK-BR-3, the BT474 cell line appears to be more sensitive to antibody treatment due to low EGFR content besides Her2 overexpression. Conclusion: The extent of decelerated or blocked cell proliferation after antibody treatment that is targeted to EGFR and to Her2 depends both on EGFR and Her2 co-expression and on antibody combination used in the treatment setting. Cetuximab did not enhance any inhibitory effect of Trastuzumab or Pertuzumab, most probably due to the dominant overexpression of Her2. Cell susceptibility to Trastuzumab/Pertuzumab, both targeted to Her2, was defined by the ratio of EGFR/Her2 co-expression.