1. Purified blood group A substance isolated from hog gastric mucin possesses the property of enhancing markedly the virulence of S. typhosa for mice.

  2. Inactivation of blood group A substance by methods which result in the loss of its ability to inhibit A isohemagglutination also results in loss of its virulence enhancement activity.

  3. The virulence-enhancing action of blood group A substance is considerably augmented by the inorganic fraction of mucin. Thus, 1.5 per cent of blood group A substance combined with 0.5 per cent mucin ash enhances the virulence of S. typhosa to a degree which compares favorably with that of 3 per cent blood group A substance or 5 per cent mucin.

  4. It is suggested that the general phenomenon of virulence enhancement by gastric mucin may be attributed to blood group A substance augmented by inorganic salts.

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