1. Normal colonic mucin is heavily sulphated and this increases its resistance to degradation by bacterial enzymes. Any defect in mucus sulphation could therefore be important in the pathogenesis of ulcerative colitis. 2. Rectal biopsies taken at colonoscopy from patients with ulcerative colitis (n = 9), patients with Crohn's disease (n = 6) and control subjects (n = 16) were cultured for 24 h in the presence of N-[3H]acetylglucosamine and [35S]sulphate. Mucin was then extracted and purified, and the ratio of [35S]sulphate to N-[3H]acetylglucosamine incorporated into pure mucin was assessed. 3. The ratio of [35S]sulphate to N-[3H]acetylglucosamine incorporated into mucin was significantly reduced in rectal biopsies taken from patients with ulcerative colitis (0.463, 0.305-0.703, geometric mean and 95% confidence intervals) compared with control subjects (0.857, 0.959-1.111, P < 0.01). In patients with Crohn's disease the reduction in this ratio (0.559, 0.378-0.829) did not quite reach statistical significance (P = 0.06). There was no difference between the ratio of [35S]sulphate to N-[3H]acetylglucosamine incorporated into mucin in Crohn's disease and that in ulcerative colitis (P = 0.26). 4. In control subjects the ratio of [35S]sulphate to N-[3H]acetylglucosamine incorporated into mucin was higher in the rectal biopsies (0.882, 0.618-1.022) than in their paired proximal colonic biopsies (0.602, 0.421-0.861; P < 0.01), but this regional variation was not observed in either ulcerative colitis (rectum: 0.450, 0.262-0.773; right colon: 0.470, 0.321-0.690, P = 0.3) or Crohn's disease (rectum: 0.459, 0.260-0.815; right colon: 0.492, 0.260-0.929, P = 0.8).(ABSTRACT TRUNCATED AT 250 WORDS)