Ubenimex (Bestatin, Ubx) has been shown to have anti-tumor activity and immuno-modulating activities. Ubx has been used in immuno-therapy in combination with remission maintenance chemotherapy after induction of complete remission for adult acute non-lymphocytic leukemia (ANLL, AML). Daunomycin (DNR), arabinosylcytosine (Ara-C) and 6-mercaptopurine (6-MP) are used for the standard chemotherapy for ANLL. It is, however, believed that emergence of resistant cells to chemotherapy cause minimal residual leukemia resulting in poor prognosis. Ubx has been administered in combination with these chemotherapeutic agents. We examined the combinatorial effect of Ubx with DNR, Ara-C, 6-MP and etoposide on K562 leukemic cell line and the chemotherapeutic agent resistant cells derived from K562 cell line. Ubx showed growth inhibitory effects on these cell lines. A synergistic effect was observed on growth inhibition and with colony formation of parent k562 cell line when DNR and Ubx were used in combination. A combination of Ubx with Ara-C or etoposide showed additional effects on parent cells and other resistant cell lines. The combined growth inhibitory effect of 6-MP and Ubx was stronger than the effect of 6-MP alone. These results show that Ubx has a direct growth inhibitory effect on leukemic cells and additional or synergistic effects are obtained on K562 leukemic cell line and on chemotherapeutic agent resistant cells derived from the K562 cell line when Ubx is used combination with the above chemotherapeutic agents.