This work reports the transmission of bovine albumin using 50k MWCO, 100k MWCO and 0.2 mu m membranes under controlled fluxes and low transmembrane pressures. With the 50k MWCO membrane, the transmission remained low and when the flux was increased step by step, there was a sharp increase in transmission as the flux reached 50 lm(-2) h(-1). The concentration of bovine albumin was estimated at the membrane surface by the classical film theory and did not increase sharply. It is suggested that increasing extensional shear at the higher flux might change the conformation of BSA molecules. This sudden change in rejection was not observed when a 0.2 mu m membrane was used. It was also observed that the transmission at iso-electric point of bovine albumin (pH 4.9) was much higher than that at either pH 3.5 or pH 8 under the same operating conditions.