We examined changes in mouse fecal microflora after various dietary components andBacillus subtilis(natto) spores were delivered by intubation. The administration of intact spores ofBacillus subtilis(natto) did not affect fecalEnterobacteriaceaeandEnterococcusspp. in all three diet groups; on the other hand, it did affect fecalBacteroidaceaeandLactobacillusspp., depending upon the diets fed. The administration of autoclaved spores did not alter fecal microflora. In vitro cultures ofLactobacillus murinusobtained from mouse feces, together withBacillus subtilis(natto) under aerobic conditions as a mixed culture, revealed that the growth ofL. murinuswas enhanced by the addition of intact spores ofBacillus subtilis(natto). This enhancement of growth was displayed only in media containing either sucrose, glucose, maltose, or fructose but not in media containing cornstarch, soluble starch, or microcrystalline cellulose. From these results it was evident that some metabolites ofBacillus subtilis(natto) produced during germination and (or) outgrowth of spores of this strain, requiring monosaccharides or oligosaccharides, participated in the enhancement of the growth ofL. murinus.Key words:Bacillus subtilis(natto),Lactobacillus, probiotics, intestinal microflora, dietary components.