This study was aimed to investigate the mechanism of indoleamine 2, 3-dioxygenase (IDO) activity in acute myeloid leukemia cells contributing to tumor immune escape. Myeloid leukemia cells were isolated from bone marrow of 23 patients with acute myeloid leukemia (AML) and IDO expression was detected by immunochemistry and RT-PCR methods. Then mixed lymphocyte reaction (MLR) of one way was carried out, leukemia cells were used as stimulating cells and T-lymphocytes were used as reactive cells in culture with or without 1-MT. T-lymphocyte proliferation rate was determined by MTT assay and IDO activity in supernatant of MLR was detected by high-performance liquid chromatography (HPLC). The results showed that IDO expression was found in 17 out of 23 cases of acute myeloid leukemia cells; IDO enzyme activity in leukemia cells inhibited T-lymphocyte proliferation in MLR cultures. It is concluded that IDO activity expressing in leukemia cells can suppress T-lymphocyte proliferation responses, which may be contributing to tumor immune escape.