The Ilex paraguariensis St. Hil. (yerba mate) saponins effectively inhibit the passive diffusion of cholic acid through dialysis membranes. The formation of large macromolecular mixed micelles between cholic acid and yerba mate saponins could explain this effect. The observed inhibition of the passive diffusion of cholic acid used as a model of bile acids, could have an antihypercholesterolaemic effect in vivo due to an increased sterols excretion. © 1997 John Wiley & Sons, Ltd.