The anemia that usually accompanies rheumatoid arthritis has been thought to be caused by an infection of unknown etiology. The author found from his experiment that a temporary anemia is caused in rabbits by the injection of the serum of patients with rheumatoid arthritis into them, but no such effect by the injection of the serum of healthy subjects. The author presumes the existance of an anemia-producing substance in the serum of the rheumatoid arthritis. The nature of this substance is now under investigation.

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