The benefits of supplementing livestock diets with extracts from Yucca schidigera have been attributed to inhibition of selected gut microbes. The antimicrobial constituents were identified as three butanol-extractable 5β-spirostan-3β-ol saponins using Bacillus pasteurii and Saccharomyces cerevisiae as test prokaryotic and eukaryotic organisms, respectively. Although these saponins inhibited microbial growth at low cell densities, their impact was associated with adsorption to the microbes and they had no effect on dense microbial populations. The antimicrobial saponins were also observed to adsorb to the microbiota and other solids of the porcine cecum. The implications for gut microbes differ according to their ecological niche: Those sequestered to feed particles or the gut lining may accumulate saponins, whereas those flushed out synchronously with the digesta should be protected by high population densities.