The author studied the mechanism of hemagglutination inhibition by organ emulsions in comparison with that of the culture media filtrate of cholera vibrio. The results are briefly summarized as follows: 1) The hemagglutination-inhibiting substance in organ emulsions acts directly on the virus. 2) The culture media filtrate of cholera vibrio inhibits the hemagglutination by acting on red cells. 3) Organ emulsions had no effect on the mouse-infectivity of influenza virus. 4) By still standing at 37℃ for 2 hours, the viruses adsorbed on organ substance refloat out. The inactivated viruses, however, do not float out by this procedure.