A bifidogenic growth stimulator was present in the cell-free filtrate of Propionibacterium freudenreichii 7025 culture and in the methanol extract fraction of the cells. Several intestinal bacteria, such as Bacteroides, Enterobacter, and Enterococcus, which also released a growth stimulator for bifidobacteria, may play an important role in regulation of a bifidobacterial population in colonic microflora. The water-soluble stimulator from the methanol extract of the cells was partially purified. The molecular weight of the stimulator appeared to be < 3000. The stimulatory activity was unaffected by treatments with pronase, carboxypeptidase A, ribonuclease, or nuclease P1 and was heat stable over a wide pH range. This stimulator differed from cyanocobalamin and from organic acids, such as acetate and propionate. Because it was stable to heat and proteolytic enzymes, the stimulator is a useful bifidogenic factor that can reach the large intestine while retaining its activity. Short-chain fatty acids were highly inhibitory to the growth of many intestinal bacteria, particularly Gram-negative facultative and obligatory anaerobes. The short-chain fatty acids (especially propionate) stimulated the growth of bifidobacteria. The growth of Bifidobacterium adolescentis 6003 was further enhanced in the presence of short-chain fatty acids and the stimulator produced by P. freudenreichii 7025. Viable counts of strain 6003 grown with Bacteroides vulgatus JCM 5826T increased more than 10(4) over those of the single culture of strain 6003. However, the growth of strain 6003 was inhibited in the mixed culture with Clostridium perfringens 7028. In continuous culture, the growth of bifidobacterial strain 6003 could be greatly enhanced, even in the presence of clostridial strain 7028, in media with short-chain fatty acids and stimulator produced by P. freudenreichii 7025.