The probiotic potential of 3 yeasts strains of Saccharomyces cerevisiae isolated from kefirs and feces was investigated and compared with 3 isolates from medicines and 2 collection strains (ATCC) of Saccharomyces cerevisiae var. boulardii. Genetic identification of yeasts based on karyotypes indicated their affiliation to Saccharomyces spp. although chromosomal polymorphism was observed. Concerning probiotic characteristics survival in simulated gastric and intestinal environment were examined. The survival of all tested yeasts in medium of pH 2.5 was comparable and equaled 86.8-97.1% after 8 hours of incubation at 37 degrees C. The fecal isolate, probiotic and collection yeasts showed also high resistance to pH 1.5 and their survival was 85.3-92.1%, whereas for kefir strains it amounted to 33.1 and 38.9%. All yeasts tested demonstrated high resistance to synthetic bile salts as well. In the presence of 0.1% sodium cholate and sodium deoxycholate the reduction of cell number by only 1 log unit after 4 hours of incubation at 37 degrees C was observed. However, 1.0% addition of ox bile did not affect their viability. In simulated gastric and intestinal environment survival of fecal, probiotic and collection strains was 86.3-93.7% after 4 hours of incubation in media with addition of 3 g/l pepsin and 1 g/l pancreatin. Kefir isolates were more sensitive to these conditions and a further 10% reduction of cell number in relation to probiotic yeasts was observed. The tested strains, except for kefir isolates, were able to grow at 37 degrees C. All the tested strains survived in sufficient number to create the possibility of proper action in the human body, although fecal, probiotic and collection strains tolerated the conditions of the human gastrointestinal tract better than food-borne yeasts.