In a semi-permeable membrane system for transferring magnetic particle fractions contained in a first fluid to a second fluid, comprising a membrane through which passages extend and which separates the first fluid from the second fluid being a pressure generation arrangement is provided for generating a reversible pressure differential between the two fluids across the membrane for alternately causing fluid movement in the membrane in opposite directions and a magnetic field generator is provided for alternately generating a magnetic field in the membrane wherein, during the fluid movement in one direction, the magnetic field is switched on to hold the magnetic particles in place in the membrane and, during fluid flow in the opposite flow direction, the magnetic field is switched off to permit the magnetic particles to be moved through the membrane in the other direction.