In a method of performing a cluster assay, a suspension (14) of superparamagnetic particles in a fluid to be analyzed is provided, wherein the superparamagnetic particles are coated with a bioactive agent. The particles are then allowed to form clusters due to an analyte present within the fluid. Subsequently, clusters of superparamagnetic particles are selectively actuated by applying a rotating magnetic field, wherein the amplitude of the magnetic field varies over time. Finally, the selectively actuated clusters are detected. An apparatus for performing a cluster assay comprises means for accommodating a sample (12) and means for applying a rotating magnetic field (11), the magnetic field being adapted for selectively actuating clusters of superparamagnetic particles. The apparatus further comprises means for detecting the selectively actuated clusters.