The effect of intraluminal challenge on rat colonic mucin producing cells and the amount and composition of released mucins was investigated. Germfree rats (GF) were maintained on a commercial high fiber (HF) diet (37% of undigestable fiber, Altromin 1640 p), in order to increase volume, dry weight and abrasive effect of the ingesta. GF control rats were fed a standard (ST) laboratory diet with 4.5% fiber (Altromin 1314 f). In the HF diet group, histological sections of the proximal and distal colon revealed a significantly increased number of mucin secreting goblet cells and an elevated goblet cell replication activity, as determined by 5'-bromo-deoxyuridine incorporation. The total amount of colonic mucins, isolated by gel filtration, was increased versus the control group. According to the results of ion exchange chromatography, carbohydrate and amino acid analysis, mucins from rats, given the HF diet, had an elevated content of acidic mucin constituents with alterations in the carbohydrate and amino acid composition. In a parallel study with specified pathogen free rats (SPF), the additional influence of the microflora on mucin secreting cells and isolated mucins was determined. An increased number of mucin secreting cells predominantly was observed in rats given the standard diet. Due to bacterial degradation, significantly less mucin was isolated from both dietary groups. The increase of acidic mucin constituents was less pronounced than in GF rat mucin, coinciding with losses of terminally linked monosaccharides. Alterations of the core protein, accompanying the presence of the microflora, were not detected.