The pumping permeate regime is a recently developed technique for minimization of fouling. However this procedure, which maintains the flux, may affect rejection. In this study a 0.22 mum microfiltration membrane was challenged with 50 nm gold sol in the pumping permeate regime below and beyond the critical flux with prefiltration of 1 mum latex beads. The rejection of colloidal gold was complete in all experiments. Above the critical flux a cake is formed and prevents the particles from passing through the membrane. Below the critical flux, with no cake formation, the particles move away from the membrane due to back-transport which is significant in the pumped permeate regime. Without movement towards the membrane, the particles do not reach it to pass through. Some of the particles which enter the membrane pores remain there due to adsorption.