The nature of platelet clump formation following massive preserved blood transfusion was studied clinically and experimentally. A heparin-like substance was increased in the plasma of recipients following massive preserved blood transfusion, which seemed to be a nature of the platelet clump formation. The platelet clump formation is a transient reversible change, and not due to antigen-anti-body reaction. It was presumed that the decrease of platelet count in recipient's peripheral blood was caused by sequestration of platelet mostly in liver, spleen, lung and bone marrow.