Experimental work on ultrafiltration is presented to illustrate the practical and theoretical principles of this separation technique. The laboratory exercise comprises experiments with pure water and with aqueous "Saccharomyces cerevisiae" (from commercial Baker's yeast) suspensions. With this work students detect the characteristic phenomena involved (polarization and gel layer formation) and measure important process variables, such as, solvent permeance, gel layer concentration, limiting fluxes, and convective mass-transfer coefficient. (Contains 5 figures.)