The secretory component (SC) polypeptide chain of secretory immunoglobulin A can be considered as a differentiation marker in that it is normally synthesized in the non-mucus-containing columnar epithelial cells, but not goblet cells, of the large intestine. With this in mind, we have studied the expression of SC in 36 colonic adenocarcinomas and 15 polyps (adenomatous and villous) by the fluorescent antibody technique. As in the normal mucosa, the synthesis of SC in tumors found in non-mucus-containing columnar cells and was absent from goblet cells. However, in several well-differentiated carcinomas it appeared that columnar cells contained both SC and mucin; these cells could be analogous to the normal mucosal precursor of both cell types. SC was synthesized throughout all adenomatous polyps and villous adenomas with the exception of some atypical nonmucinous areas of adenomatous polyps. Secretory component synthesis by carcinomas was associated with mucus production, although goblet cells did not contain SC. The presence of SC also correlated with the degree of differentiation. Secretory component was absent from half of the carcinomas as well as from atypical nonmucinous areas of polyps, and this could represent one of the earliest changes associated with the development of malignancy.