The establishment of a mucin-degrading intestinal microflora was followed in 30 healthy Swedish children by agar gel electrophoresis of fecal samples taken at 0, 1, 3, 6, 9, 12, 15, 18, 21, and 24 months of age. At 3 months, 18 children showed a limited degree of mucin degradation, while 12 children showed no degradation. In children exclusively breast-fed for at least 4 months, mucin degradation was initiated significantly later than in children who received formula, with or without breast milk, by 1 month of age. Complete degradation of mucin was seen in 21 children after 1 year of age and in all children at 2 years. A positive correlation between increasing age and increased degradation of mucin was found between birth and 1 month and between 6 and 9 months, after standardization for the period of exposure to foods other than breast milk.