Abstract: A device for manipulating and mixing magnetic particles (3) in a surrounding liquid medium, comprises at least one couple of magnetic poles (1,1?) facing each other across a gap, the facing poles diverging from a narrow end of the gap to a large end of the gap, the poles (1,1?) forming part of an electromagnetic circuit and being arranged to provide a magnetic field gradient in the gap region; and a reaction chamber (2) that is a part of a fluidic network for containing the said magnetic particles in suspension and placed in the gap of the said electromagnets poles (1,1?). The reaction chamber (2) preferably has at least one part which has a diverging cavity, arranged co-divergently in the diverging gap between the poles.