Monoclonal antibodies have been generated using membrane-enriched extracts of human metastatic mammary carcinoma lesions (Colcher et al., 1981), some of which demonstrated binding to the surface of human colon carcinoma cell lines. We report here an analysis of the reactivity of three of these monoclonal antibodies with formalin fixed tissue sections of human colon adenocarcinomas and adenomas. The three monoclonals employed were B72.3, which is reactive with a 220-400 kdal high molecular weight glycoprotein complex; B6.2, reactive with a 90 kdal glycoprotein, and B1.1, which is reactive with the 180 kdal glycoprotein CEA. B1.1 was least selective in its reactivity to colon carcinoma versus adenoma lesions. When 10 micrograms/ml of purified B1.1 IgG were used per slide, 94% (15 of 16) of carcinomas and 83% (15 of 18) adenomas showed reactivity. Monoclonal B72.3 demonstrated the most selective reactivity for carcinomas. Eighty-two percent (14 of 17) of carcinomas were positive while none of 18 adenomas examined showed reactivity with more than a few percent of adenoma cells positive. When a low concentration of purified B72.3 immunoglobulin was used per slide, 8 of 16 carcinomas and none of 46 adenomas or normal colon epithelium samples scored positive. Monoclonal B72.3 also reacted with cells in areas of "atypia" within adenomas. The reactivity of monoclonal B6.2 was intermediate as compared to B1.1 and B72.3 in its selectivity of reactivity for carcinoma cells. A heterogeneity in the populations of tumor cells showing reactivity with the three monoclonals was observed within many of the tumor masses. Both colon adenocarcinomas and adenomas can now be placed in several distinct groups based on their expression of antigens reactive with the three monoclonal antibodies employed.