It has been demonstrated that many multiples of minimal doses of Bacillus typhosus reacting factors can be neutralized by specific immune sera.

The potency of a given serum can be conveniently titrated against increasing amounts of reacting factors.

If the immune serum is diluted or if the amount of the reacting factors is too large for a given amount of serum, there is obtained neutralization but only irregularly.

Normal and heterologous sera (therapeutic meningococcus and erysipelas horse sera) free of normal agglutinins or possessing normal agglutinins of a low titer (1:16) for Bacillus typhosus are not able to neutralize the reacting factors. There is obtained questionable neutralization with a serum possessing normal Bacillus typhosus agglutinins in dilution 1:64.

The titer of the neutralizing antibodies increases in the course of immunization.

Immune sera exercise a definite protection against the mortality induced by intravenous injection of Bacillus typhosus culture filtrates.

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