Glycoprotein synthesis and secretion were measured in short-term organ culture of normal and neoplastic colonic mucosa from 11 patients undergoing colectomy for colon cancer. Mucosal explants were incubated for up to 24 hr with [3H]glucosamine, which was incorporated into both explant and secreted glycoproteins. Structural and functional viability was documented by morphological studies that showed excellent preservation of architectural detail and biochemical studies that documented a steady increase in glycoprotein synthesis during 24-hr incubation. The major difference between normal and neoplastic mucosa was a 35% decrease (p less than 0.02) in the incorporation of [3H]glucosamine into tumor explants, as compared to the amount incorporated into normal explants from the same patient. The rate of secretion of radiolabeled glycoproteins into the medium did not differ significantly. Separation of explants into particulate and cytosol fractions showed that the overall decreases in glycoprotein synthesis in tumor explants was primarily due to a marked reduction in particulate glycoprotein synthesis in the cancer tissue.