The platelet-clumping substance appeared statistically significant (p < 0.01 ∼ 0.05) in the heparinized plasma of 29 rabbits collected 5 minutes, 30 minutes, and 1 and 2 hours after the intravenous injection of either 5 ml./Kg. of 0.5 per cent agar-saline solution, 100 µg./Kg. of bacterial endotoxin derived from Escherichia coli, or 10 µg./Kg. of adrenaline, as compared with that in the platelet-saline suspension from the other 36 rabbits. At the same time, the circulating platelet count decreased significantly (p<0.01 ∼0.05). Twenty-four hours after the injection these changes disappeared. The platelet-clumping substance was detected only in heparinized plasma, and not in citrated, oxalated, and EDTA-plasma or serum. The addition or removal of calcium or magnesium ions did not affect the appearance of clumping of platelets. It is stable for storage at 4 C. and is not dialyzable. It induced clumping of platelets not only in the platelet-saline suspension, but also in the platelet-rich plasma, while ADP, serotonin, catecholamine or thrombin (0.001 ∼ 100µg./ml. or units/ml.) did not induce clumping of washed rabbit platelets even under the presence of the normal platelet-free heparinized plasma.