an apparatus is provided for quantitatively measuring groups of magnetic particles . the particles are complexed with substances to be determined and are excited in a magnetic field . the magnetizations of the magnetic particles are thereby caused to oscillate at the excitation frequency in the manner of a dipole to create their own fields . these fields are inductively coupled to at least one sensor such as sensing coils fabricated in a gradiometer configuration . the output signals from the sensing coils are appropriately amplified and processed to provide useful output indications .