The purpose of this study was to determine if a panel of monoclonal antibodies could define phenotypic markers that could be used in risk assessment of a spectrum of colonic polyps and colon cancers. Using the ABC immunoperoxidase technique on formalin-fixed sections of surgical specimens, the following results were obtained: 1) Mab B72.3 demonstrated increased reactivity in villous lesions and cancers compared with hyperplastic polyps and tubular adenomas; 2) Mab anti-CAA demonstrated increased reactivity in polyps compared with colon cancers; and 3) using the two antibodies (Mab B72.3 and Mab anti-CAA), a malignancy ratio was obtained that determined malignancy risk for individual polyps. No hyperplastic polyp gave a positive ratio, but about 30 percent of villous lesions were positive. Over 50 percent of villous lesions greater than 2 cm in size had a positive ratio. The malignancy potential ratio may be a valuable marker in assessing risk of malignancy in an individual case.