Objective To investigate whether the Toxoplasma gondii can inhibit proliferation of human leukemia K562 cells and/or induce apoptosis of the cells in vitro. Methods K562 cells (5?104/ml) were harvested at mid-ex-ponential phase and planted in 96 well plates with 100 ?l each and in 50 ml culture bottles, 1.5 ml each. The cells were treated for 48 hours with different concentration of Toxoplasma tachyzoites. Growth inhibition rate was measured with MTT method. Apoptosis was detected through following ways: fluorescence microscopy with Hoechst 33 258 staining was used for observing the change of cell morphology, agarose electrophoresis was used to detect the DNA changes and FCM was used to observe sub-diploid. Results Toxoplasma can inhibit proliferation of K562 cells. K562 cells treated with Toxoplasma presented an inhibition rate of 17%, 28%, 48%, 50% and 55% under the tachyzoite concentration of 1?104, 2?104, 4?104, 8?104 and 16?104/ml respectively, with a significant difference to the control (t=3.606, 5.918, P