A device for measuring a quantity of a superparamagnetic material comprises four coils (1-4) and means 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.
