Abstract: Monoclonal antibodies manifesting specific reactivity to benign adenomatous polyps of the human colon are produced. Hybridoma technology was utilized to produce immunoglobulin producing cell lines. Splenic lymphocytes from mice, immunized with a membrane fraction derived from human tubular adenomatous colonic polyps, were fused with a mouse non immunoglobulin secreting myeloma cell line (NS-1). Screening of hybridoma culture supernatant for positive reactivity with adenomatous colonic polyp extract, and negative activity with extracts from normal liver and spleen allowed for selection of desired immunoglobulin producing cultures. Further screening of expanded cultures with extracts of colorectal cancer, apparently normal mucosa adjacent to the cancer, and a variety of normal human extracts allowed further distinctions in binding reactivity. One such selected cell line produced a monoclonal antibody designated Adnab-9 selected on this basis.