We have observed that high-resolution ultrasonic imaging can be used to detect and measure the size and sedimentation of large human platelet aggregates formed in vitro. At progressively lower shear rates, individual platelet aggregates grew in cross-sectional area from the limit of ultrasonic detectability of less than 1 mm2 at moderate shear rates to an approximate mean area of 25 mm2 at zero shear. Platelet aggregates tended to sediment at mean shear rates higher than zero shear. At a shear rate of 5.5 s-1, 70% of platelet aggregates had settled, and at 1.6 s-1, 90% had settled to the dependent half of horizontal tubes in less than 15 s. Ultrasonic aggregometry of platelets may be a valuable technique for investigation of platelet dynamics under controlled flow conditions in vitro.