An analytical instrument for the immobilization of antigens to be detected in an immunoassay using mangetizable beads. The instrument includes a probe containing magnetizable particles and a plurality of magnetic devices for applying a magnetic field gradient to the particles in the probe reservoir. The probe is movable between the plurality of magnetic devices and at least one of the probe positions may be used to immobilize the magnetic particles in the fluid medium and at least another position may be used to resuspend the particles in a fluid medium.