The Middlebrook-Dubos hemagglutination test with normal red cells and the Boyden hemagglutination test with red cells tanned with tannic acid were shown to be strictly specific, for tubercle polysaccharide and for the protein antigens. The former test detects polysaccharide antibodies and the latter protein antibodies. Convincing evidence is given that polysaccharide does not adhere to the surface of tanned red cells nor protein to the normal red cell surface. Hemosensitizing ability and in vitro antigenicity were found to be two distinct properties of tubercle polysaccharide and protein antigens.

The phosphatide hemagglutination test with normal red cells sensitized with tubercle phosphatide is also specific; it detects only phosphatide antibodies.

The three different kinds of antibodies, antipolysaccharide, antiprotein, and antiphosphatide, were shown to be completely distinct from each other in tuberculous serum. Each of them can be completely removed from serum by absorption with red cells or kaolin particles coated with its corresponding antigen.

The reliability of the cross-inhibition test is discussed.

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