Abstract: An apparatus for detecting the presence of magnetic particles in a fluid is described. The apparatus uses a magnet which is mounted so that its field is transverse to the fluid flow, so that the magnetic field imparts a transverse velocity to any magnetic particles in the fluid. An ultrasound transducer mounted adjacent to the magnet. The ultrasound transducer is used to direct ultrasound energy into the moving fluid and to detect any particles having a velocity which is not in the direction of the main fluid flow, i.e., the velocity of a particle affected by the magnetic field will be detected by the ultrasound transducer as a result of the Doppler shift in the transmitted and reflected ultrasound frequency.