The paper describes the mechanism of virus (poliovirus) removal from water using 0.2 mu m microfiltration and 30 kD cutoff ultrafiltration membranes. It was found that in microfiltration of poliovirus, standard blocking or the adsorption of viruses to the membrane matrix is the dominant mechanism. When mixed with E. coli the initial adsorption is followed by cake formation. In ultrafiltration of polioviruses or mixtures of E. cell and poliovirus, cake formation is the dominant mechanism.