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The surface coat of human platelets was stained with positive charged colloidal Thorotrast particles and ruthenium red to elucidate the mechanism of platelet aggregation by macromolecules.  Positive charged macromolecule, polylysine (Mw.l5,000; 23,000; 180,000) could induce the aggregation in low concentration but high concentration was needed in the case of neutral macromolecule, dextran (Mw.40,000; 250,000; 2000,000). The larger molecules of polylysine and dextran were more effective in inducing platelet aggregation. In the dextran induced aggregation, Thorotrast particles on the cell surface did not decrease significantly. On the other hand, the surface membranes of platelets treated with neuraminidase and aggregated by polylysine were essentially devoid of bound particles. These findings suggested that polylysine induced aggregation more effectively than dextran by reducing the negative surface charge and giving stronger adsorption force on cell surface.  In washed platelet suspension, polylysine induced aggregation without release reaction of 14C-serotonin, but average distance between plasma membranes of aggregated platelets did not vary with the degrees of polymerization. In this respect, it seems probable that after forming macromolecular bridging of cell surface, another interaction may follow between cell surfaces.

Ultrastructural Studies on the Surface Coat of Human Platelets Aggregated by Polylysine and Dextran