A system is developed to magnetically measure biological antigen-antibody reactions with a superconducting quantum interference device (SQUID) magnetometer. In this system, antibodies are labeled with magnetic nanoparticles of γ-Fe 2 O 3 , and the antigen-antibody reactions are measured by detecting the magnetic field from the magnetic nanoparticles. A setup of the system is described, and the sensitivity of the system is studied in terms of detectable weight of nanoparticles. Magnetic particles as small as 600 pg can be detected at present. An experiment is also conducted to measure antigen-antibody reaction with the present system. It is shown that the sensitivity of the present system is better than that of the conventional optical method. A one order of magnitude improvement of sensitivity will be realized by the sophistication of the present system.