Sixty-four breast-fed infants and 23 calves were investigated for bacteria and enzymatic activity in their faecal samples. The bacteria were measured using cultivation and fluorescence in situ hybridization. Enzymatic activity was also examined. Forty-seven (64%) infants and all the calves had high numbers of bifidobacteria (usually >9 log CFU g-1) in their faeces, but 17 infants (36%) did not have a detectable amount of the bacteria. Most of the bifidobacteria-negative infants had significant quantities of clostridia in their faecal flora. While the infants did not have significantly higher counts of bifidobacteria, the samples from calves contained significantly (P<0.05) more coliform bacteria and lactobacilli. There were also significant differences in their enzymatic activities. Bifidobacteria-positive samples had a greater alpha-glucosidase activity, while bifidobacteria-negative samples had a lower activity of alpha-galactosidase, and calf samples had the highest beta-glucuronidase activity. A significant increase in bifidobacteria in calf faeces between days 3 and 7 was accompanied by a decrease in Escherichia coli. Our results show that the faecal flora of calves is similar to that of infants with regard to the occurrence of bifidobacteria as a dominant bacterial group.