A method for separating, or removing, particulate material, e.g., blood cells, from a sample of fluid, e.g., whole blood of a patient, in which the particulate material is suspended. In the case of separating blood cells from blood plasma or blood serum, the resulting samples of blood plasma or blood serum can be used for in vitro diagnostic applications. In normal practice, a whole blood sample of a patient are provided and then introduced into an apparatus that contains a flow channel. An acoustic field, which contains acoustic standing waves from external ultrasonic transducers, is located within the flow channel. Laminar flow is maintained in the flow channel. Blood cells and platelets are separated from blood plasma or blood serum at the end of the flow channel and collected. The method described herein allows fluid components to differentially migrate to areas of preferred acoustic interaction. The parameters that affect separation of particles are size, density, compressibility of the particles, and the fluid surrounding the particles.