Aims: The objectives of this study are to investigate the fate of a streptomycin-rifampicin-resistant variant of Bifidobacterium longum SBT2928 (BL2928SR) and the influence of its oral administration on the composition and metabolism of the intestinal microflora. Methods and Results: Intestinal passage of BL2928SR was monitored by a combination of selection with antibiotics and identification by a randomly amplified polymorphic DNA (RAPD)-PCR method. Intestinal microflora was analysed by the method developed by Mitsuoka et nl. (1965, 1974). Long-term survival of orally administered BL2928SR in the human intestine was confirmed. BL2928SR ingestion specifically lowered faecal populations of Enterobacteriaceae and clostridia, including lecithinase-positive Clostridium spp. Conclusions: BL2928SR and its parent strain, BL2928, are considered to be appropriate candidates for probiotics. Significance and Impact of the Study: It is clarified that BL2928SR has the ability for long-term survival in the human gastrointestinal tract, and alters the composition and metabolism of the intestinal microflora.