Sialylation of mucus glycoproteins confers charge and increased resistance to enzymatic degradation. The hypothesis that mucus sialylation might be altered in ulcerative colitis has been studied using in vitro culture of mucosal biopsies for 24 h with 3H N-acetyl mannosamine as a specific sialic acid precursor. Rectal biopsies were obtained at colonoscopy from patients with clinically inactive ulcerative colitis (n = 9) and controls (n = 12) who were patients found to have a normal colonoscopy performed for iron deficiency anaemia or altered bowel habit. The incorporation of 3H N-acetyl mannosamine into mucin was increased in ulcerative colitis (n = 9; 150; 113.3-393.2 dpm/micrograms mucin, median and interquartile ranges), compared with controls (n = 12; 33.6; 19.7-68.4 dpm/micrograms; p < 0.01). The ratio of incorporation into mucin of 3H N-acetyl mannosamine/14C N-acetyl galactosamine was also increased in ulcerative colitis (3.27; 1.93-4.98 dpm/dpm), compared with controls (1.35; 1.24-1.7 dpm/dpm; p < 0.001) suggesting that the increased incorporation of N-acetyl mannosamine probably reflects an increase in the average extent of sialylation per mucin oligosaccharide chain rather than an increase in the number of oligosaccharide chains. This increase in mucin sialylation seems unlikely to have a pathogenic role in the development of colitis but provides further evidence for the similarity between the alterations that occur in ulcerative colitis, colonic polyposis and malignancy.