Probiotics are initially defined as food supplements containing live microorganisms which beneficially affect the host by improving intestinal microbial balance. Although the definition has been expanded by other researchers from time to time, in this view, lactic acid bacteria (LAB) are unalterably considered to be the main components of probiotics as they fulfill, in the best course, the requirements targeted by the basic idea of these expanded definitions. It has been believed that periodical supplementation of special cultures of probiotic LAB is helpful to keep the person who consumes it in good health. Recently, the ingestion of LAB as a probiotic has drawn much interest all over the world because of growing health consciousness and concern. A typical example is seen in Japanese society. Well-being is one of the most important matters of concern for Japanese because of the rapid aging of the society. The beneficial image of probiotics has led to an increase in the consumption of fermented milk products in Japan as well as in other countries. Bifidobacteria are often used as dietary supplements or as starter cultures in the production of fermented milk products used as health foods. I have developed the probiotic strain Bifidobacterium longum SBT2928 (BL2928) over a decade for this purpose. This review summarizes the research done to demonstrate the biological functions of BL2928, especially regarding its effect on the host's immune system, on intestinal transit as well as the intestinal microflora composition and metabolism, and specific competitive exclusion of enterotoxigenic Escherichia coli by a novel anti-infectious factor (BIF).