This paper reports the performance of a new plasma Filter which consists of a tubular membrane containing a concentric screw-threaded insert. Bovine blood was used as the working fluid, which was pumped through the annular space between the insert and the tubular membrane in oscillatory flow with a mean component. The effects of pulsation frequency and piston stroke length were investigated. Membranes of 0.2 and 0.45 mu m pore sizes were tested. For the range of Strouhal number 0.0018-0.0003 in the predominantly laminar flow regime, it was found that there was a linear relationship between the bovine plasma flux and the inverse of the Strouhal number. Haemolysis, measured by the concentration of free plasma haemoglobin, proved to be insignificant. Due to the depolarizing influence of the flow pattern induced by pulsations, protein recovery was close to 100%.