The formation of a two-dimensional aggregate of 25 mu m latex particles in a 1.5 MHz ultrasound standing wave (USW) field and its disintegration in a flow were studied. The aggregate was held in the pressure node plane, which allowed continuous microscope observation and video recording of the processes. The trajectories and velocities of the particles approaching the formation site were analyzed by particle image velocimetry (PIV). Since the direct radiation force on the particles dominated the drag due to acoustic streaming, the acoustic pressure profile in the vicinity of the aggregate was quantifiable. The drag coefficients D-coef for 2- to 485-particle aggregates were estimated from the balance of the drag force F-D and the buoyancy-corrected gravitational force during sedimentation on termination of the ultrasound when the long axis of the aggregate was in the vertical plane. D-coef were calculated from F-D as proportional to the aggregate velocity. Experiments on particle detachment by flow (in-plane velocity measured by PIV) from horizontal aggregates suspended in deionized water and CaCl2 solution of different concentrations showed that the mechanical strength of the aggregates depended on the acoustic pressure amplitude P-0 and ionic strength of the solution. In deionized water the flow velocity required to detach the first single particle from an aggregate increased from 1 mm s(-1) at P-0 = 0.6 MPa to 4.2 mm s(-1) at P-0 = 1.4 MPa. The balance of forces acting on particles in a USW trap is discussed. The magnitude of the shear stress employed (similar to 0.05 Pa) and separation forces suggests that this technique can be applied to studying the mechanical responses of cell aggregates to hydrodynamic flow, where cell-cell interaction can be separated from the effects of solid substrata.