Isogalactobiose (β-D-galactopyranosyl β-D-galactopyranoside), galsucrose (β-D-fructofuranosyl α-D-galactopyranoside) and lactosucrose (O-β-D-galactopyranosyl-(1→4)-O-α-D-glucopyranosyl β-D-fructofuranoside) were synthesized as potential sugar sources that selectively enhance the growth of bifidobacteria in the human intestines. Isogalactobiose was synthesized by means of the Koenigs-Knorr reaction and the others by using levansucrase. The structures of these sugars were confirmed by enzymic hydrolysis. All of the sugars synthesized were utilized by bifidobacteria, but not by strains of Lactobacillus acidophilus, Streptococcus faecalis and Escherichia coli. However, isogalactobiose was utilized by 65%, galsucrose by 62% and lactosucrose by 41% of thirty-seven strains of Enterobacteriaceae isolated from nineteen adults, while raffinose, which had seemed to be the best among commercially available sugars tested, was utilized by 49% of the strains.