Secretory immunoglobulins are found in nongoblet columnar cells of normal intestinal epithelium. These molecules consist of a secretory component portion, which is synthesized in the columnar cells, and an immunoglobulin portion which enters the columnar cells from plasma cells in the adjacent lamina propria. In the present work, the synthesis and transport of these various subunits have been studied by immunofluorescence in benign polyps and cancers of the colon. In both the epithelium and plasma cells of benign and malignant tumors, as well as in normal tissue, IgA is the principal immunoglobulin, followed by IgM. However, when compared to normal tissue, neoplastic epithelium contains less immunoglobulin and also less secretory component; the decrement usually inversely parallels the degree of differentiation. Thus, benign polyps closely resemble normal colonic mucosa in so far as the secretory immunoglobulin system is concerned. In contrast, atypical areas of benign polyps and carcinomas exhibit greatly decreased or absent synthesis and transport of secretory IgA. Plasma cells tend to be markedly decreased in the stroma of carcinomas, suggestive of an alteration in the normal mechanism for attracting the circulating precursors of local IgA plasma cells. Whenever neoplastic epithelium contained IgA, plasma cells with IgA could be observed in the vicinity; this is in keeping with the concept of local synthesis of secretory IgA. In some instances in which local plasma cells were plentiful, neoplastic cells were deficient in secretory component and IgA, which suggested impairment of the mechanisms for transporting IgA across epithelium. The possible role of secretory component in such transport and in attracting lymphoblasts to mucous membranes is dicussed.