Villin is a cytoskeletal protein of microvilli of epithelial cell brush borders found principally in absorptive cells of the intestine and proximal renal tubule. A marker of both enterocyte differentiation and epithelial cell polarity, it has been studied mainly in experimental animals. We raised monoclonal antibodies to villin and used them to localize it in human ileum and colon and in 22 colonic neoplasms. Villin is localized in the brush border of normal ileum and in the luminal border of normal colon and is expressed with increasing staining intensity as cells migrate from crypt to surface. It was present in the luminal border in all five adenomas and in 16 of 17 adenocarcinomas studied. In addition, villin staining was observed in the cytoplasm of 10 tumors, and in the basement membrane area surrounding tumor in 10 cases. In "transitional" mucosa adjacent to carcinomas it was confined to the luminal border. Abnormal expression of villin by a significant proportion of colonic tumors suggests that it may have a role as a marker of colorectal neoplasia.