The manipulation of small (<5-μm-diam) eukaryotic cells and of bacteria (ca. 1.5-μm diameter) has been examined here in a number of batch and flow systems containing ultrasonic standing waves. The The different systems, involving plane and tubular ultrasonic transducers and a range of acoustic pathlengths, have been designed to treat suspension volumes of analytical scale, i.e., 5 ml to 50 μl for both single-sample and ‘‘on-line’’ situations. The frequencies employed range from 1–12 MHz. Removal efficiencies of 99.5% have been achieved for yeast and for the bacterium Escherichia coli. The influence of secondary cell–cell interaction forces in determining the cell concentration dependence of harvesting efficiency in batch sedimentation systems is considered. The contributions of pulsing and of field modulation techniques to achieving high harvesting efficiencies are illustrated. Applications of standing wave radiation forces to (i) levitate cells in suspension for microscopic examination of cell interactions; (ii) to enhance particle agglutination immunoassay detection of cells or cellular products, and (iii) accelerate the aqueous biphasic separation of cells are presented. [Work supported by BBSRC and EU.]