Differential light scattering cuvettes for the measurement of thromboemboli in high shear blood flow systems. Newly developed optical scattering cuvettes were constructed as a modification of our existing 1.0 mm and 3.0 mm internal diameter (ID) cuvettes to facilitate the measurement of platelet microemboli ranging from 20 microns to 1,000 microns diameter in whole blood in 0.9 mm ID flows ranging from 250 to 4,000 ml/min. A perturbation solution to the one-speed radiative transport equation was used in the design and calibration of these cuvettes. A series of tests were performed with these cuvettes in an extracorporeal left ventricular assist device bovine model, and in a recirculating closed-loop flow system containing anticoagulated whole baboon blood, to determine to what extent they affect platelet and erythrocyte function ex vivo and in vitro. Serial hemolysis tests, thromboxane radioimmunoassay measurements, platelet counts, and activated partial thromboplastin times were measured. All of these tests with cuvettes in the extracorporeal and in vitro circuits were statistically indistinguishable from baseline measurements, suggesting the usefulness of this system for the measurement of microemboli in blood-contacting materials of extracorporeal circuits and cardiac assist devices. 