Local secretion of complement components in the human intestine has been previously reported. However, the cellular source has not been identified. In this study, we demonstrate complement C3 and factor B mRNA expression in the normal colonic mucosa by in situ hybridization analysis. C3 and factor B genes were found to be expressed at high levels in the epithelial cells of the lower parts of the crypts in colonic mucosa, and this expression decreased gradually from the crypt base to the luminal surface. At the upper crypt and the luminal surface, these genes almost disappeared. C3 and factor B genes were expressed in all crypts at the same level. Furthermore, C3 and factor B gene expression was also identified in adenomas and carcinomas. In these neoplastic tissues, C3 and factor B genes were expressed uniformly, and the polarized distribution observed in the normal crypts was not detected. It is likely that complement components are locally synthesized in the intestine, and that these complement components may actively participate in normal immune and inflammatory responses over the enormous surface area of the intestinal mucosa.