A new method for the detection of magnetically labeled cells in suspension using an optical microscope is presented. The movement of individual cells in a nonuniform magnetic field (magnetophoresis) can be studied and the direction and velocity of their movement can be determined. Cell-associated magnetic particles (e.g., attached magnetic immunomicrospheres) influence these parameters. We developed a magnetophoresis chamber for measuring the movement of single cells in a high-gradient magnetic field. Paramagnetic erythrocytes were used to monitor the level of magnetic force in the chamber. Bw6 antigen-positive REH cells were used as a biological model. Magnetic microspheres coupled with anti-Bw6 antibody were used as magnetic labels. The movement of magnetically labeled REH cells was investigated versus the intensity of magnetic field. The magnetization of microspheres was experimentally determined by the Faraday balance method. Experimental and theoretical results substantiate the feasibility of detecting a single cell associated with a single magnetic microsphere. Thus, magnetophoresis provides a sensitive biometrical system for magnetically labeled entities. o 1994 Wiley-Liss, Inc.