The separation of alpha-lactalbumin (alpha-La) and beta-lactoglobulin (beta-Lg) from casein micelles during microfiltration of skim milk was investigated using a dynamic filtration pilot (MSD) equipped with six rotating ceramic membranes of 0.2 mu m pores. Permeate fluxes at initial concentration and 40 degrees C reached a plateau with transmembrane pressure at 100 kPa, ranging from 62 L h(-1) m(-2) at a rotation speed of 1044 rpm to 120 L h(-1) m(-2) at 1930 rpm. Casein rejection remained higher than 95% and increased at low speeds. alpha-La and beta-Lg transmissions increased with increasing rotation speed, ranging from 38% to 49% for alpha-La and from 25% to 40% for beta-Lg. For comparison, similar tests were carried out with a filtration module consisting in a disk equipped with 6 mm high vanes rotating at high speed near a fixed 15.5 cm diameter PVDF membrane of 0.15 mu m pores. Permeate fluxes were higher than for ceramic disks, reaching 215 L h(-1) m(-2) at a disk rotation speed of 2500 rpm and 150 L h(-1) m(-2) at 1500 rpm. Casein rejection was slightly higher than for ceramic membranes, exceeding 97.5% and reaching 99.5% at 2500 rpm. However, alpha-La and beta-Lg transmissions were lower than for the ceramic disks, dropping to 10% above 150 kPa and 2000 rpm for alpha-La and to 7% for beta-Lg. Rotating ceramic membranes offered the best compromise between flux and whey protein transmission and can be an alternative to co-current recirculation of permeate in MF of milk.