A slightly modified Salzman technique for platelet adhesiveness is described and a normal range of 32% to 65% has been established. There is no variation with age or sex. But investigation shows that this type of technique is sensitive to variations in the packed-red-cell volume of the blood, and that for comparable estimates the PCV must be between 33% and 46%. The platelet adhesiveness test gives a low result in von Willebrand’s disease and low results have also been found in 4 syndromes characterized by defective platelet aggregation. The reasons for correlation of platelet adhesiveness and platelet aggregation, the limitations of the use of the platelet adhesiveness as a screening test, and the possible role of a plasma factor are discussed.