Fresh human platelet‐rich plasma was prepared from adult blood anticoagulated with EDTA, and the platelets allowed to incorporate tritiated serotonin. Samples were sheared for 5 min at 23°C by the acoustic microstreaming field generated around submerged portions of a transversely oscillating steel wire (115‐μm radius driven at 20 kHz). Immediately following irradiation, each sample was centrifuged for 5 min to sediment intact cells and debris, and the quantity of serotonin released into the supernatant was measured using a liquid scintillation counter. There was no measurable threshold below which serotonin was not liberated, after allowances had been made for the spontaneous release of serotonin. In addition, it was found that the amount of serotonin released by irradiation alone varied with the square of the wire‐displacement amplitude, indicating that hydrodynamic shear stress and velocity gradient are probably the agents responsible for the release of serotonin (together with the simultaneous release of clot promoting and/or accelerating factors).