Objective: To explore the antitumor mechanisms of bifidobacteria adolescence in vivo. Methods: The content of extracellular signal regulated proteins (ERK)1/2, C-Jun N-terminal kinase (JNK), p38, c-fos and c-jun in nude mouse transplanted large bowel carcinoma was detected by using laser confocal microscopy. The expression of NF-kB was determined by immunohistochemistry. Results: After the nude mouse transplanted tumor was treated with bifidobacteria, the average fluorescent strength of ERK1/2, JNK, c-fos and c-jun was significantly lower than that in tumor control group (P0.05). The positive cell density of NF-kB in large bowel carcinoma transplantation tumors in Bifidobacterium injection group was markedly lower than that in tumor group(P