Bifidobacterium sp is a natural component of the dominant colonic microflora that was recently introduced into several fermented dairy products. The aim of the present study was to study the fate of this microorganism in the human gut. On the basis of antibiotic resistance characters, a variant of Bifidobacterium sp that could be distinguished from indigenous bifidobacteria in the fecal flora was selected, and its survival and colonization in the colon was examined. This strain was used to ferment milk, and 125 g of the fermented product obtained was ingested by eight healthy volunteers three times daily for 8 days. Stools were recovered and weighed throughout the study. The results showed that the exogenous Bifidobacterium sp appeared in the stools and reached a mean level of 8.8 +/- 0.1 log colony-forming units per gram. This level was maintained as long as the fermented dairy product was consumed. When its ingestion stopped, the exogenous Bifidobacterium sp gradually decreased and was no longer detectable 8 days after cessation. The mean recovered quantity during the 8-day period of administration of the ingested bifidobacteria excreted in stools was 12.1 +/- 0.1 log colony-forming units per gram, i.e., 29.7% +/- 6% of the ingested bacteria, which was similar to the percentage that reached the colon in previous studies. It is concluded that under physiological conditions, exogenously administered Bifidobacterium sp do not colonize the human colon. However, the high fecal concentrations of exogenous bifidobacteria reached are compatible with metabolic "probiotic" activities.