An apparatus for retaining magnetic particles in self-assembled magnetic particle structures in a liquid flow, comprises a micro-channel (5) and magnets (1,2) for generating a substantially static magnetic field across the micro-channel (5) such that magnetic particles (12) suspended in a liquid in the micro-channel (5) form magnetic particle structures (15). The micro-channel (5) has along its length transverse large sections (8) alternating with narrow sections (9). The large sections (8) are periodically distributed along and on either side of the narrow sections (9) so that in use magnetic particle structures (15) form across the large sections (8) of the micro-channel (5), the magnetic particle structures being retained by engagement of end parts (18) of the magnetic particle structures in the large sections (8) of the cell or channel (5). The apparatus is useful in particular in life science, chemistry and microfiltration applications by flowing through the micro-channel (5) a fluid carrying molecules or particles to be captured, filtered or activated by the magnetic structures (15).