Probiotic microorganisms to be used as biotherapeutic agents have to resist the rigors of the upper human gastrointestinal tract. In this study we evaluated the acid tolerance in vitro and the fecal recovery in vivo after oral administration of a Saccharomyces cerevisiae strain to healthy volunteers. At the lowest pH value (pH 1.0) the yeast load in tablets decreased slightly. From pH 1.0 to pH 7.0 the release of S. cerevisiae in buffer solutions increased. The selected yeast strain showed good tolerance to low pH which mimic the gastric environment. After one month of treatment at a dose of 100 million cells per day, S. cerevisiae grew from the feces of 6 (37.5%) of the 16 healthy, treated volunteers. Based on the results of the present experiments the yeast studied can be considered a strain that tolerates adverse conditions comparable to those of the human gastrointestinal tract, and when administered orally may colonize the bowel of healthy volunteers and even replace resident Candida species.