The monoclonal antibody MC_3 related antigen was expressed in colon carcinoma, its transitional mucosa, colon polyp and fetal colon mucosa with immunohistochemical staining.The positive rate was 95.38%,47.00%,70.83%, 65.08%, 69.23%, 42.00%, and 37.50% in colon carcinoma, transitional mucosa, fetal colon mucosa, familial polyp, villus adenoma, tubular adenoma and infantile polyp respectively. Moreover, the positive rate became higher as the proli-ferative grading of polyps was larger.The findings suggest that monoclonal antibody MC_3 can be used as a means to detect the possible cancerous degeneration of colon polyps and transitional mucosa of the colon can be considered as a precancerous lesion.