Human fecal samples were used as a source of Bifidobacterium strains which are resistant to both acid and bile. The procedure used for screening was as follows: enrichment of Bifidobacterium strains with Bifidobacterium-selective transgalacto-oligosaccharide-propionate (TP) medium followed by acid (pH 2.0) and bile salt stressing (1.5% bile salt (w/v)). Two selected Bifidobacterium strains, designated HJ 30 and SI 31, showed considerably higher rates of survival when incubated in 50 mM phosphate buffer solution adjusted to pH 2.0 or 3.0 or in 50 mM phosphate buffer (pH 7.0) containing 0.5 or 1.0% (w/v) bile salt. HJ 30 and SI 31 were the only strains to have significant growth in Man Rogosa Sharpe (MRS) medium at 0.15% bile salt. All strains tested had similar growth rates in the absence of bile or at an initial pH value of 5.0 or 7.0 as determined by optical density measurements. For SI 31 the number of viable cell counts remained high (6 x 10(7) cfu/ml) for up to 72 h when grown in the skim milk medium, whereas all other strains examined declined to below 10(5) cfu/ml. These results demonstrate that the screening procedures developed in this study are effective for the selection of acid and bile resistant Bifidobacterium strains.