Weekly faecal specimens from 18 babies were examined during the first 8 weeks of life. Eight infants were breast fed, ten were bottle-fed. All suckling infants received supplementary feeds for the first 8 days. A buffer consisting of acetic acid and acetate was demonstrated in the faeces of all the breast-fed infants at some time during the period of examination. This buffer was rarely detected during the 1st week of life when supplementary feeds were given, and buffer already present gradually disappeared with the introduction of mixed feeding. In contrast, at no time was an acetate buffer demonstrated in the faeces of bottle-fed infants. Babies receiving breast milk produced faeces with low pH, high counts of saccharolytic organisms including bifidobacteria and Streptococcus faecium, and low counts of Escherichia coli, bacteroides and clostridia. Bottle-fed infants on the other hand produced faeces with a high pH and high counts of E. coli and putrefactive bacteria, but with low counts of bifidobacteria.