Nonintrusive manipulation by standing-wave acoustics has been widely used for levitation and trapping of macroscopic objects.1 The standing-wave field creates an acoustic radiation force on the object that depends on its size and acoustic parameters.2 In the present article we demonstrate size-selective separation and retention of latex spheres inside a small-diameter flow-through capillary by use of an ultrasonic radiation trap. The work aims at rapid in-flow detection and separation of specific molecules via antibody-coated latex spheres. Acoustic radiation forces have been used for nonintrusive manipulation of macroscopic as well as microscopic objects. Basically, objects with higher acoustic impedance than the surrounding medium are trapped in the velocity antinodes of the standing-wave acoustic field. The theory of acoustic levitation and trapping is well understood.2,3 In aqueous solutions the method has been applied for studies of mechanical properties of mm- to mm-sized liquid droplets and biological cells.4,5Â¦..