We examined the effect of bestatin (Ubenimex) on the growth of human leukemic cells (i.e, HL-60, K562, MT-1, MT-2, Molt-4, and Raji cells). The growth of each cell line was inhibited by the cocultivation with bestatin at higher concentrations than employed for clinical use in Japan. [3H]TdR incorporation was also inhibited in MT-1 and MT-2 cells by treatment with bestatin. Degenerated cell-to-cell adhesion was observed among the treated cells. These findings suggest that the inhibitory effect on some leukemic cells, especially on MT-1 cells, results from the inhibition of DNA synthesis.