Objective: To explore the antitumor mechanisms of whole peptidoglycan of bifidobacterium. Methods: The apoptotic cells and the positive expression of bcl-2 and bax oncoprotein were studied nude mice transplantation tumors of colorectal carcinoma by employing in situ end labeling technique and immunohistochemical staining. Results:The apoptotic cell density, the positive rate and the staining intensity of bax oncoprotein of the transplantation tumor of colorectal carcinoma in the whole peptidoglycan injection group were significantly higher when compared with the tumor control group. The positive rate of bcl-2 oncoprotein in the whole peptidoglycan injection group was obviously lower than that in the tumor control group (P＜0.01).Conclusion: Whole peptidoglycan of Bifidobacterium bifidum could induce cell apoptosis of nude mice transplantation tumors of colorectal carcinoma by downregulating the expression of the bcl-2 gene and upregulating the expression of the bax gene.