The prolonged extraction of the tubercle bacillus with boiling ethyl alcohol, followed by one or more reprecipitations by chilling the hot alcoholic solution, easily yields a preparation very active as antigen in the complement fixation reaction.

This preparation gives a precipitation reaction with high dilution of the normal blood serum of a number of species.

The precipitation reaction presents as a peculiar feature a very long pro-zone and is further dependent on a preceding heat treatment of the serum for its demonstration.

Occurring as a reaction of normal serum, the reaction is apparently not influenced by immunization sufficient to develop moderate specific complement fixation reactions.

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