1. The addition of small amounts of cholesterol and of cephalin reduces markedly the protective action of antipneumococcus horse serum.

2. These lipids do not affect the protective action of antipneumococcus rabbit serum.

3. These findings may be explained (a)by the selective adsorption of lipid on the antigen-antibody complex, and (b) by certain lipid antagonisms.

4. The failure of large amounts of immune horse serum to protect mice against pneumococcus infection is explicable on the basis of selective participation of lipids dependent upon the species from which the antibody is derived.

5. The lipids modify the results of protection tests only through participation in the process of specific sensitization.

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