This study was purposed to investigate the growth inhibition and apoptosis-inducing effect of Clostridium difficile toxin A (TcdA) on the leukemia cell line K562. The proliferative activity of K562 cells exposed to Tcd A was tested by MTT assay, cell apoptosis was detected by flow cytometry; immunocytochemistry and colorimetric assay were employed to detect the protein expressions of BCL-2/BAX and the activity of Caspase-3, respectively. The results indicated that the proliferation of K562 cells was inhibited in a time-and dose-dependent manner after exposure to Tcd A for 24, 48 and 72 hours, the cells displayed the typical apoptotic, morphological changes, the expression of BCL-2 protein was down-regulated but the expression of BAX protein was signficantly increased, compared with control group (p < 0.05). In addition, caspase-3 was activated in a concentration-dependent manner. It is concluded that Tcd A inhibits cell growth of K562 by inducing apoptosis, and the up-regulation of BAX protein and activation of caspase-3 may play important roles in these processes.