Galacto-oligosaccharides (GOS) are considered to have bifidogenic properties in humans. To study the effect of GOS-containing syrup (60% GOS) alone or together with the probiotic strain Bifidobacterium lactis Bb-12 on selected components of the faecal flora, and the effect of GOS supplementation on colonisation of B. lactis Bb-12, a feeding trial on 30 healthy volunteers was performed. Mean numbers of bifidobacteria increased slightly in all study groups during the feeding period. Isolates having the identical RAPD genotype with B. lactis Bb-12 were detected in high numbers in the Bb-12 group and in the GOS-containing syrup+Bb-12 group indicating a good survival of B. lactis Bb-12 through the gastrointestinal tract. No differences in the prevalence or numbers of isolates with B. lactis Bb-12 genotype could be observed between groups suggesting that GOS-containing syrup did not enhance the survival or persistence of B. lactis Bb-12 in the gut.