Perfusion studies with human blood are traditionally performed in relatively large chambers (10x 0.6 mm) through which blood is circulated at 55 mL/min to achieve a shear rate (s.r.) of 1600/sec. A volume of 15 mL is sometimes used which means the blood is recirculated up to 20 times through the chamber during a standard perfusion of five minutes. We have developed a new chamber of much smaller dimensions (5x0.1 mm) through which blood is drawn with a syringe pump at a velocity of 0.3 mL/min for a s.r. of 1600/sec. This chamber has the advantage that little platelet activation occurs during perfusion. Smaller quantities of inhibitors or antibodies are required and it is possible to perform several perfusions with native blood from a single patient. The setup, advantages and disadvantages of the system are described. A modification of the same chamber has been used for the direct observation and quantitation of the adhesion of neutrophils and monocytes to platelets and/or fibrin under flow. Platelet adhesion under flow has also been studied with direct observation using this same chamber.