In this paper there are reported observations on a certain new reactivating property of normal and immune blood sera of various animal species. The effect of the reactivating sera consists in restoration of the toxicity in vivo and in vitro of completely neutralized meningococcus and B. coli reacting factors.

The property is apparently non-specific, heat-labile, lessens on storage, and has no relationship to complement.

Heating of immune sera to a temperature destructive for the reactivating property but innocuous to the neutralizing antibodies, raises considerably their neutralizing potency.

The possible immunological significance of the reactivating property is discussed in this paper.

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