1. The injection of antigen into an animal determines a gradual change in the blood fluid which finds expression in two physicochemical manifestations that can readily be followed, namely a decrease in the static value of the surface tension of serum solutions, and a special form of crystallization when serum diluted with isotonic sodium chloride solution is allowed to evaporate under certain conditions.

2. The change in the blood is at a maximum around the 13th day after the first antigen injection, and decreases progressively thereafter until it can no longer be observed, which is usually around the 30th day.

3. The change follows the same course, whether a single large injection of antigen is made, or many smaller ones. It begins at the same time in either case, it comes to a maximum after the same period, and in its subsequent course it is not affected by the reinjection of antigen. The manifestations of the change would appear to be independent of the presence of antigen in the circulation.

4. The mean length of the protein molecules of the immune serum obtained after the injection of the antigen dealt with is little if at all different from that of the protein molecules of normal serum.

5. It is possible that this reaction is independent of the antibody formation.

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