The present application is directed to the separation of--a solid fraction from a fluid sample particularly a therapeutic cellular fraction from a biological sample such as a bone marrow sample by a porous filter (2) which separates a filtration unit (1) into an upper pre-filtration chamber (3) into which a fluid sample (4) requiring cell separation is introduced and a lower post-filtration chamber (5) into which a fluid (6) capable of transmitting an acoustic standing wave is introduced. An acoustic element (8) is coupled to a substrate (7) which is located within and at the bottom of the lower chamber (5) and which resonates in response to the acoustic generating element (8) and generates a standing wave through the two fluid phases and the filter to agitate the sample (4). Simultaneously, a cyclic process of vacuum draw (9). causes movement of the sample (4) downwards through the filter (2). Vacuum pressure, fluid flow rate and frequency of vibration are controlled from a remote unit housing appropriate pumps and valves.