Objective: The ATP-binding cassette sub-family G member 2 ABCG2 (BCRP) is a major multi drug resistance (MDR)-related protein that is frequently overexpressed in cancer patients. Recent reports indicate a link between cyclooxygenase-2 (COX-2) and development of the multidrug resistance phenotype. In this study, we aimed to evaluate the influence of 12-O-tetradecanoylphorbol-13-acetate (TPA) as a COX-2 inducer on ABCG2 expression and activity in human breast cancer cell lines (MCF-7, MCF7-MX and MDA-MB-231). Materials and Methods: The cytotoxicity of TPA was determined by MTT assay. The effects of TPA on ABCG2 mRNA and protein expression were studied using real-time RT-PCR method and flow cytometry assay, respectively. Its modification of ABCG2 protein activity was also investigated by flow cytometric mitoxantrone efflux assay. Results: TPA exhibited very little growth inhibitory activity in all tested cell lines. ABCG2 and COX-2 were highly expressed in MCF7-MX and MDA-MB-231 cells, respectively. Treatment of MDA-MB-231 cells with TPA increased expression of COX-2 up to 11-fold to control level. TPA caused a considerable increase up to 9-fold in ABCG2 mRNA expression in parental MCF-7 cells, while a slight increase in ABCG2 expression was observed in the resistant cell line MCF7-MX. Flow cytometry analyses of ABCG2 protein expression confirmed the real-time PCR results and showed a positive correlation between ABCG2 protein expression and COX-2 protein level in each cell line. Moreover, TPA could increase ABCG2 activity in all cell lines with the greatest stimulatory effects in MCF7-MX (more than 6 times the control level). Conclusion: Our findings provide evidence that TPA induces ABCG2 expression and activity in drug sensitive MCF-7 breast cancer cell line which proposes the existence of a positive correlation between the expression of COX-2 and ABCG2 genes.