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Notes: Abstract Background: A significant proportion of breast cancer patients receiving tamoxifen therapy relapse during treatment following acquisition of tamoxifen-resistant or oestrogen-independent phenotypes. The mechanism behind this rapid progression to oestrogen autonomy is at present unclear and further treatment modalities are limited. Suramin represents a novel potential second line therapy. The mechanism of the antineoplastic activity of suramin is not completely understood, although the drug binds to many growth factors including epidermal growth factor and insulin-like growth factors and can also dissociate growth factors from their receptors. In this study we have related suramin sensitivity to the expression of receptors for epidermal growth factor and insulin-like growth factor-I in a number of breast cancer cell lines including lines resistant to tamoxifen. Materials and methods: The anti-proliferative effects of suramin were investigated in two oestrogen dependent breast cancer lines (ZR-75-1 and MCF-7), oestrogen independent (ZR-PR-LT) and tamoxifen resistant (ZR-75-9a1) variants of ZR-75-1 and a tamoxifen resistant (LY2) variant of MCF-7. Full dose response curves were constructed and IC50values determined for each cell line. Sensitivity to suramin was correlated with the level of expressio n of receptors for epidermal growth factor (EGFR) and insulin-like growth factor-I (IGFR). On observing stimulation of cell proliferation by suramin in the tamoxifen resistant cell lines in the presence of tamoxifen we also investigated the possible role of suramin sequestration of transforming growth factor-β in mediating this effect. Results: All cell lines exhibited a dose- and time-dependent response to suramin treatment. Tamoxifen resistant ZR-75-9a1 cells (day 6 IC5085 µg ml−1) were more resistant to suramin than oestrogen independent ZR-PR-LT cells (day 6 IC5045 µg ml−1), and the parent ZR-75-1 line (day 6 IC5056 µg ml−1). Increased sensitivity to suramin was associated with increased expression of IGFR and decreased expression of EGFR. Tamoxifen resistant LY2 cells were significantly more sensitive to suramin (day 6 IC5070 µg ml−1) than MCF-7 cells (day 6 IC50350 µg ml−1). Both IGFR and EGFR expression by LY2 cells was lower than in the parent line. The antioestrogen-resistant ZR-75-9a1 and LY2 lines grown in the presence of 8 µM tamoxifen were growth stimulated by concentrations of the drug below 100 µg/ml. As growth stimulation observed in the presence of tamoxifen may have been due to suramin sequestration of tamoxifen induced TGF-β1 secretion we also investigated the response of the cells to this peptide in the presence and absence of suramin. All cell lines were growth inhibited by TGF-β1 except ZR-75-9a1 which was unresponsive. Responses to TGF-β1 were modified in the presence of 100 µg suramin ml−1 although TGF-β1 was unable to mimic the ability of tamoxifen to stimulate proliferation in the presence of suramin. Conclusions: These results suggest that for ZR-75-1 cells and variants, increased sensitivity to suramin is associated with an increase in expression of IGFR and a decrease in EGFR numbers. However, tamoxifen resistant LY2 cells, in which both IGFR and EGFR expression is reduced were considerably more sensitive than parental MCF-7 cells suggesting that there is no clear relationship between EGFR and IGFR expression and suramin sensitivity. The unexpected stimulation of cell proliferation of the tamoxifen resistant variants by suramin in the presence of tamoxifen could not be explained by suramin sequestration of transforming growth factor-β and the mechanism of this interaction remains unclear.