To study the cytotoxicity and mechanism of cytokine-induced killer (CIK) cells to breast cancer cell line ZK-75-1.The morphological changes of ZK-75-1 cells were observed by HE staining. The apoptosis of ZK-75-1 cell line was examined by TUNEL staining. The expression of P53, P16, C-myc, Bcl-2 and Bax was determined by immunocytochemical staining.The result of HE staining revealed that CIK cells moved toward the target cells, forming typical rosette cells. Granule-like substances appeared in cytoplasm of tumor cells, but only granule-shape patch was found in some tumor cells while breast cancer cells as control grew well. TUNEL staining indicated that the cells in control group were not stained or dyed well-distributed light blue. As the cells in experiment group became smaller, mucleoluser or perinuclear was dyed deep blue. The apoptotic rate of ZK-75-1 cells cocultured with CIK cells was increased after 4-12 hours and was decreased after 12-24 hours, with a significant difference compared with control group (P<0.01). Immunocytochemical staining showed that the expression of p53, p16, C-myc and Bcl-2 proteins in CIK group declined but the expression of Bax protein increased with the passage of time, which was significantly different compared with control (P<0.01).The mechanism of killing ZK-75-1 cells by CIK cells is closely related to the downregulation of the expression of P53, P16, c-myc and Bcl-2 proteins and to the upregulation of the expression of Bax protein. It also has close relation with the time of exposure to CIK cells.