Abstract: Separation apparatus and method for separating magnetic and/or magnetically-labeled particles from a test medium in a reaction chamber which incorporates baffles, a spinner, a stirrer, or a plunger operable to be displaced along a collection surface to define a narrow flow path through which the particle-laden test medium must flow. The collection surface is a thin-walled non-magnetic material having a plurality of magnetic pole faces positioned therearound. Test medium within a reaction chamber is caused to flow past a collecting surface, and a high-gradient magnetic field is applied to the surface to capture magnetically responsive particles in the test medium. The particles are deflected toward the collection surface in the flow path for the test medium. The narrow flow path assures that substantially all of the particles pass within the high-gradient magnetic field along the collection surface.