S ynthesis and secretion of colonic mucin glycoprotein species were assessed during in vitro culture of colonic mucosal explants. DEAE-cellulose chromatography of endogenously labeled mucin glycoproteins from explant tissue demonstrated the presence of six mucin species (I-VI) similar to those identified earlier in surgical specimens of human colonic tissue. The relative proportions of mucin species I-VI in tissue explants remained constant throughout a 30-h culture period. However, the proportional representation of the various mucin species in media was significantly different from that found in tissue, which suggests that some mucin species (I, II, and III) are differentially secreted, whereas others (IV and V) are retained within intracellular pools. Radiolabeled precursors were incorporated into mucin species I, II, and III at a 2.0-2.6-fold greater rate than their concentration in tissue, supporting the concept that these glycoproteins were both synthesized and secreted at a greater rate than species IV and V. Colonic mucosal explants from patients with ulcerative colitis showed greater than 90% reduction of species IV. However, the amount of species IV recovered from culture media of ulcerative colitis explants was comparable to normal controls. It appears that mucin species IV is differentially secreted rather than retained within intracellular pools in mucosa of patients with ulcerative colitis.