During microbial colonization, mucin-releasing goblet cells of germ-free (GF) rats proliferate and upregulate their mucin synthesis, thus improving the intestinal mucus barrier. The present study determined the significance of bacterial membrane constituents for this development. A single dose of lipopolysaccharide (LPS) (35 micrograms/100 g body weight) and lipid A (3.5 micrograms/100 g body weight, respectively), was perorally administered to GF AS/Ztm rats. One, 3 and 5 days later, sections of the proximal and distal colon served for characterization of mucin-secreting goblet cells, released mucins were isolated in parallel. Maximal goblet cell diameters were evidenced at day 3. LPS generated a maximal goblet cell hyperplasia one day after challenge, lipid A stimulated the goblet cell proliferation continuously up to day 5. Three days after challenge with one of the stimuli, either, intracellular mucins had shifted significantly to neutral constituents. In addition, mucins, adherent to the colon mucosa and submerged to the luminal content, respectively, then were augmented. At day 5, adherent mucins were similar to the controls, while luminal, soluble constituents had further increased. Histometrical and biochemical methods evidenced a transient, inflammatory response of mucin-secreting cells, followed by an upregulated release of immature mucins.