With the use of various animal organs, the author studied the inhibition to the hemagglutination reaction by influenza virus. The results are summarized as follows: 1) In all of the mouse, the rat and the guinea-pig, and of all the organs tested, the brain. the lung, the liver, the kidney and the spleen, the lung showed the highest and the brain the lowest hemagglutination inhibition. 2) The inhibition titer diminished by heating; in the mouse lung emulsion and in the fowl liver emulsion, heating at 80℃ for 30 minutes and that at 70℃ for 30 minutes respectively destroyed the hemagglutinationinhibitory activity nearly completely. 3) In the range from pH 5.9 to 8.0, the fowl liver emulsion showed a stable hemagglutination inhibition. 4) In the virus inactivated by heating at 56℃ for 30 minutes, by adding 2,000×merzonin (kept refrigerated for 1 week) or by adding formalin (kept refrigerated for 1 week), no change was observed in the hemagglutination inhibition. 5) The hemagglutination-inhibiting substance was not filtrable through Seitz E. K. 6) Freezing and thawing of organ emulsions produced no effect on their hemagglutination inhibitory activity. 7) Sensitization of red cells with organ emulsions caused no change in the hemagglutination inhibition; the organ emulsions directly act on the virus.