Mucin degradation was studied with one Clostridium (RS42) and two Bacteroides (RS2 and RS13) strains isolated from the pig colon mucosa. Mucins from pig colon and stomach were prepared in their subunit forms for use as growth substrates, and the loss of the individual sugars from the mucins was measured after bacterial growth. Colonic mucin was more resistant to degradation than gastric mucin. The strains differed in their competence in degrading the mucins. Carbohydrate plus sulfate removal from gastric mucin varied from 63 to 76% for RS2, 37 to 46% for RS13, and 37 to 53% for RS42. All three strains removed more fucose (67 to 87%) and less sulfate (22 to 63%) than the average carbohydrate plus sulfate loss. Under the same conditions of growth, a mixed pig fecal culture removed 78% of sulfate and 96% of each sugar. Of the two major glycoprotein types present in the subunit pig gastric mucin preparation (R. A. Stanley, S. P. Lee, and A. M. Roberton, Biochim. Biophys. Acta 760:262-269, 1983), the less highly sulfated mucin was more susceptible to RS42 degradation. The degradation of gastric mucin by RS2 was not affected by glucose or high sulfate concentrations in the growth medium. The results show that the three strains of colon bacteria are capable of significant hydrolysis of mucin carbohydrate and that the extent of degradation seen with pure cultures is determined in part by the subunit glycoprotein type(s) present in the mucin.