We measured fecal pH, volatile fatty acids (VFA), and lactic acid concentrations in 67 healthy infants ranging in age from 1 to 5 months. The babies were divided into five groups according to their diet: group 1: human milk, with subgroups of 1 month of age (n = 8), and 2-5 months (n = 12); group 2: formula milk, with subgroups of 1 month of age (n = 9) and 2-5 months (n = 10); group 3: cow's milk, one group of 2-5 months of age (n = 12); group 4: human milk supplemented with formula milk, 2-5 months of age (n = 8); and group 5: human milk supplemented with cow's milk, 2-5 months of age (n = 8). Fecal VFA concentration and pH of bottle-fed infants (groups 2 and 3) were significantly higher and lactic acid concentrations significantly lower than those of the breast-fed group. Groups 4 and 5 showed significantly higher pH and propionic acid concentration, and lower lactic acid concentration than Group 1 did. There were no significant differences in the data on pH, VFA, and lactic acid concentrations between the feces of formula and cow's milk groups. On the other hand, the addition of formula or cow's milk to the human milk was sufficient to alter the profile of VFA, the pH, and the lactic acid concentration of breast-fed infants stools. Low pH, low VFA, and high lactic acid concentrations make the colonic content of breast-fed babies an environment favorable for Bifidobacteria growth, and this is probably the main protective factor against gastrointestinal infections.