There is great interest in the possibility of increasing numbers of bifidobacteria in the intestine from the standpoint of nutrition and health in various mammals including humans. Natural rubber serum powder (NRSP), was found to have a remarkable growth-promoting effect on various kinds of microorganism, in particular, anaerobes. NRSP stimulated the growth of all the species of Bifidobactem’um employed and synergism was noted between the effects of NRSP and nutrients in media that contained yeast extract, meat extract, and casein as nutrients.