A mixed culture of human fecal bacteria was grown for 120 days in a three-stage continuous culture system. To reproduce some of the nutritional and pH characteristics of the large gut, each vessel had a different operating volume (0.3, 0.5, and 0.8 liter) and pH (6.0, 6.5, and 7.0). A mixture of polysaccharides and proteins was used as carbon and nitrogen sources. Measurements of H2, CH4, S2-, sulfate reduction rates, sulfate-reducing bacteria (SRB), and volatile fatty acids were made throughout the experiment. After 48 days of running, porcine gastric mucin (5.8 g/day) was independently fed to vessel 1 of the multichamber system. The mucin was extensively degraded as evidenced by the stimulation of volatile fatty acid production. In the absence of mucin, sulfate-reducing activity was comparatively insignificant and methanogenesis was the major route for the disposal of electrons. The reverse occurred upon the addition of mucin; sulfate reduction predominated and methanogenesis was completely inhibited. This was attributed to release of sulfate from the mucin which enabled SRB to outcompete methanogenic bacteria for H2. SRB stimulated by mucin were acetate-utilizing Desulfobacter spp., lactate- and H2-utilizing Desulfovibrio spp., and propionate-utilizing Desulfobulbus spp. When the mucin pump was switched off, the multichamber system reverted to a state close to its original equilibrium. These data provide further evidence that sulfated polysaccharides such as mucin may be a source of sulfate for SRB in the human large gut.