Using HT-29 colonic cancer cell line as the immunogen, monoclonal antibody Hb3 was generated, which can react to colonic carcinoma and several colonic cancer cell lines. In this paper, formalin fixed tissue sections of the human colonic cancer and adenoma were tested by indirect immunofluorescence using monoclonal antibody Hb3. In 20 colonic cancers, 16 belonged to well and moderately differentiated carcinoma, 4 to poorly differentiated carcinoma and mucoid adenocarcinoma. All of the former reacted to monoclonal antibody Hb3 but no reaction was seen in the latter. The positive rate was 80%. By the same method, 20 colonic adenomas were also studied. 19 were negative except one, a papillo-adenoma with dysplasia, which was weakly positive. The results indicate that monoclonal antibody Hb3 can be used in differentiating colonic carcinoma from adenoma and as an effective indicator of the malignant change of colonic adenoma.