The author studied the inhibitory action of nucleic acid to hemagglutination by influenza virus. Just like animal organ emulsions, nucleic acids (commercially obtained) inhibited the hemagglutination by influenza virus, and the viruses adsorbed on ribonucleic acid re-floated out by still standing at 37℃ for 2 hours. These results suggest that the hemagglultination-inhibiting substance in organ emlsions is ribonucleic acid or its relating compounds.