Abstract: A one-quarter wave separation chamber of an ultrasound resonator is effective at particle/fluid/living cell separation, and can integrate with a sterile-disposable kit. Resonator is part of a continuous flow closed system, and can replace the centrifuge for blood separation into constituents. Resonator is optically monitored for cell type/volume near each exit port to control outlet valves maintaining collection purity. Uncollected cells/plasma can return to donor patient. Apheresis benefits: less extra corporeal blood volume, decreased processing time, smaller system size, lower instrument cost, reduced haemolysis, and lower cost kits. The separation chamber of the kit can be efficiently coupled acoustically to the resonator body using evacuating sealing gaskets surrounding the interface. Cooling medium can flow through resonator counteracting temperature increase tendency with higher power applications.