Saccharomyces boulardii (SB) is a yeast that is used for the prevention and treatment of antibiotic-associated diarrhea and for the treatment of pseudomembranous colitis. Since SB will most commonly be used when the bacterial flora of the gastrointestinal tract have been disrupted by antibiotic treatment, the influence of different antibiotics on the kinetics and recovery of SB in feces was investigated in rats. Following a single oral dose, SB concentrations in feces were measured for periods of 1 to 6 days. Although SB is eliminated exclusively in the feces, less than 3% of the dose is recovered as viable yeast. When rats were treated with neomycin, which is active against gram-negative aerobic bacteria but not against anaerobes, no change was observed in recovery of SB when compared with recovery from untreated rats. Also, there was no change in the rate at which SB concentrations declined in feces. In contrast, treatment with clindamycin and the broad-spectrum antibiotic ampicillin, which are active against anaerobes, produced an increase in the recovery of SB of up to seven times that of controls and slowed the rate of decline of SB concentration in the feces. This antibiotic effect on SB disposition was also found when SB was administered in multiple doses. An eightfold increase in the steady-state output of SB was observed from ampicillin-treated animals. Analysis of the recovery and kinetic data showed that the primary effect of these antibiotics was to reduce the destruction of SB, probably in the cecum and colon.(ABSTRACT TRUNCATED AT 250 WORDS)