Part 1 of this study concerns the dependence of shear-induced platelet aggregation on platelet concentration. Uniform shear stesses of 5, 75, 1, 15, 2, and 25 dynes per square centimeter were applied to platelet suspensions by Couette flow viscometry for 3 seconds. Relative-Platelet-Aggregation (RPA) is defined as the number of platelets aggregated during shear divided by the number present before shear. Whole blood from a group of normal, healthy, non-smoking human subjects was centrifuged to prepare platelet plasma with various platelet concentrations. Aggregation was determined by electronic particle counts both before and after the specimens were exposured to shear in the viscometer. RPA was found to be proportional to initial platelet concentration at all shear stresses studied. RPA increased with shear stress up to 15 dynes per square centimeter, where RPA was a maximum (82 percent for a platelet concentration of 31, per microliter). Shear stresses higher that 15 resulted in lower levels of aggregation (57 percent at a stress of 25). Part IX outlines the procedures and equipment needed to investigate shear stress-induced platelet reactions at very small times of specimen exposure to shear. A review of the role of ionic calcium as a platelet activator is presented. Internal platelet calcium can be localized and quantitated by electron microscopy. Measurement of calcium redistribution can be used as an aggregation stimulation indicator. A capillary viscometer-nitrogen freezer apparatus and freeze drying system were designed and constructed to prepare specimen for these purposes.