A device for measuring a quantity of a superparamagnetic material includes four coils (1-4) and a unit for injecting, into the coils, a direct current and two alternating currents of different frequencies. The quantity of superparamagnetic material is derived from an amplitude of a component at a mixing frequency, which frequency is a linear combination of the frequencies of the two alternating currents. The device is designed to reduce the influence of stray electromagnetic fields and inadvertent signal offsets on the measurement results. The device may advantageously be used for immunoassays.
