ABCG2 is a protein that belongs to the ABC protein superfamily, in which some proteins, including ABCG2, contribute to the phenomenon called multidrug-resistance (MDR), a resistance to wide range of drugs during cancer or AIDS chemotherapy. Identifying the substrates and inhibitors of ABCG2, and understanding its working mechanism is of crucial importance for overcoming drug resistance. In this study we examined the effect of several known modulators and selected polyphenols on the transport activity of ABCG2. Known ABCG2 ligands such as quercetin, tariquidar and Ko143 inhibited the transport activity of the protein, while Cyclosporin A had a weak effect on the ABCG2 dependent mitoxantrone (MX) transport. We also demonstrated that the previously untested compound N-ethylmaleimide (NEM) is also a strong inhibitor of ABCG2. In addition, we tested numerous dietary polyphenols and found them ineffective on the transport activity of ABCG2.