Sera produced by immunization with crystalline oxyhemoglobin react species-specifically with hemoglobin solutions.

Evidence is presented that in this reaction the hemoglobin itself is the active substance.

Conversion of oxyhemoglobin into methemoglobin, carbon-monoxy hemoglobin, or cyanhemoglobin does not alter the response to the precipitating immune serum.

Not only the hemoglobin of homologous species, but also that of other species causes inhibition in greater or less degree of the precipitin reaction.

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