The monoclonal antibody AM-3 was raised against mucins extracted from human colorectal carcinomas. It reacted strongly with sections of paraffin wax embedded colorectal carcinoma. In colonic adenoma tissue the percentage of cells expressing the epitope detected by AM-3 correlated with the degree of dysplasia. In contrast to immunohistochemical staining, which did not show the presence of the antigen in histologically normal mucosa, the more sensitive enzyme linked immunosorbent assay (ELISA) and immunoblot assays showed that it was weakly expressed in this tissue. AM-3 reacted with variable frequency with several normal and malignant human tissues, indicating that the detected epitope is not restricted to colonic tissue. In colonic carcinomas it is present on a sialomucin of apparent relative molecular mass of more than 440,000. These data suggest that the antigen detectable with AM-3 may be useful in the assessment of premalignant changes in colonic adenomas.