Membranes are finding increasing application in disinfection processes for raw water and municipal effluent reuse. The efficiency of virus removal is vitally important if membranes are to become widespread in these applications. In particular the ability of microfiltration (MF) to adequately retain viruses is of interest. This paper describes experimental studies of poliovirus transmission through 0.2 mu m microfiltration (MF) and 30 kD cut-off ultrafiltration (UF) membranes. Transmission was measured for both dead end and stirred conditions and for transmembrane pressure values of 25, 50, 100 and 200 kPa. Feed solutions containing virus only, virus in the presence of biomass (E. coli) and virus in the presence of turbidity were used. It was found that UF membranes give complete rejection of virus and that MF membranes gave significant removals under appropriate conditions.