Introduction. As regards the composition of the precipitate, a great deal has been written, and there has been considerable discussion as to whether it is composed wholly of precipitinogen, or wholly of precipitin, or in part of both. That the last named is the correct view of its composition will, I think, be evident from a large number of experiments included in the present study. In order to establish this fact at the outset, however, the following experiment is inserted:

Experiment. To 1 cc. of Serum 894, rabbit versus horse serum, is added 0.02 cc. of horse serum. The resulting precipitate is thoroughly washed, and is then injected intraperitoneally into a guinea pig. Four animals are prepared in this manner. After four days, two of these pigs are given an intravenous injection of 0.5 cc. of horse serum. Both die in anaphylactic convulsions. This demonstrates by the method of passive sensitization the presence of rabbit-versus-horse antibodies in the precipitate.

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