1. Solutions rich in antitoxin were obtained by a process of partial peptic digestion. The antigenic qualities of such “modified” preparations were highly impaired.

  2. A small fraction of unmodified protein seems to account for the residual antigenic activity.

  3. The antibody itself in the digested preparations is present in a modified form which has lost its antigenic qualities.

  4. Previous observations that antitoxins in unmodified form have the same antigenic qualities as the protein of the species from which they are derived, are confirmed.

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