To enable the real-time observation of the platelet adhesion onto biomaterials under shear flow conditions, we equipped a cone-plate type viscometer with a transparent cone and bottom plate. A transparent test material (56-66 mm ) was placed as a bottom plate of the viscometer and the whole apparatus was mounted on the stage of an inverted microscope.Washed platelet suspension (4.0 10 5 platelets/μl) was prepared from anesthetized rabbits. Three test materials were chosen as typical artificial biomaterials: polyvinyl chloride, polycarbonate and acrylate resin. Time course of platelet adhesion onto the test materials was recorded for 30 min on video tapes by a TV camera in shear rates of 3, 30 and 300 l/s. The numbers of platelets adherent to the test materials were counted manually and theoretically analyzed on the basis of convective diffusion formula (Feuerstein et al., Trans ASAIO, 21, 427-435, 1975) with 2 parameters of the effective platelet diffusivity (De) and the surface reactivity constant (K).Using this apparatus, microscopic images of platelet adhesions to the test materials under a shear flow were clearly visualized at real-time. Material under the high shear rate, 300 l/s, showed the lowest adhesiveness of the platelets. Acrylate resin which has the smallest water contact angle showed the lowest adhesiveness among the three materials. Agreement between experimental data and simulated values by calculating the above formula seemed satisfactory. K increased in materials with large contact angle and under high shear rate, while De increased in materials with large contact angle and under low shear rate. Consequently, our cone-plate viscometer-based apparatus should become an important instrument for screening biomaterials that provides a precise quantitative evaluation of platelet adhesion.