The resistance of tumor cells to chemotherapy is a major problem in the treatment of tumors. The most investigated mechanism of resistance is the increased excretion of the drug from cells by the ABC transporter. They normally participate in the excretion of various toxins and xenobiotics, thus protecting the cell from adverse effects. However, their overexpression on the surface of the tumor cells leads to resistance and a reduced response to treatment. ABCG2 is a 72 kDa protein from the ABC transporter family that has a wide variety of substrates, including many known drugs. Given that ABCG2 can significantly modulate treatment efficacy, it is important to investigate its interactions with compounds. In this study, we investigated the effects of six primaquine derivatives (primaquine and halogenaniline fumardiamides and bis-urea primaquine derivatives), synthesized as potential antimalarials and/or anticancer drugs at the Faculty of Pharmacy and Biochemistry in Zagreb, on ABCG2 transporter activity. As a model, we used a cell line of acute myeloid leukemia without (PLB) and with increased expression of ABCG2 (PLB-G2), which shows resistance to various substrates of this pump, such as mitoxantrone. We tested the antiproliferative properties of the compounds using the MTT assay, while the inhibition of ABCG2 was tested with the Hoechst 33342 substrate accumulation assay and with 5D3 shift assay, by flow cytometry, using Ko143 as a positive control. The compounds with the strongest activity were tested with a combination treatment with mitoxantrone, to verify possible enhancement of mitoxantrone activity in resistant cells.