To evaluate the commercial potential of new microbial feed additive, Issatchenkia orientalis Y266 and Bacillus subtilis B266 from commercial fermented rice bran were tested for their tolerance or resistance to pH, bile, oxgall, and temperature. It was found that the strains grew very well up to pH 3.0 and resistant to relatively high concentrations of bile salt and oxgall. I. orientalis and B. subtilis are extremely tolerant in range of 70-90 degrees C in solid medium. B. subtilis B266 also has excellent tolerant property up to 90 degrees C in liquid medium. The health indexes (the microflora in the small intestines and the antibody titer to Newcastle disease virus) of chicks were significantly improved in the fermented rice bran with these strains (0.25% addition to diet) in comparison with the Avilamycin (20 mg/kg diet)-fed group (p<0.05). The fermented rice bran-fed group showed a better microbial flora in the small intestines. Accordingly, it would appear that the fermented rice bran with these strains may be a potential candidate for an alternative microbial feed additive.