Abstract: A sample of liquid with suspended particles is contained in a tube and subjected to a standing wave ultrasound field transverse to the tube, the standing wave exhibiting a progressive change in pressure amplitude transverse to the tube, so that particles in suspension are displaced transversely of the tube to one or more predetermined regions; exposure of the sample to the standing wave is then terminated and the particles are allowed to settle, and inspected to determine whether they remain aggregated or whether they dissociate. The ultrasound field is produced by a transducer of tubular form encircling the tube. The invention may be used for agglutination of particles or cells via cross-bridging molecules in immuno-agglutination assays.