A soluble specific substance was isolated from Mexican typhus Rickettsia which gave, with Proteus X-19 antiserum and typhus human serum, the same precipitation reactions as the polysaccharides extracted from B. proteus OX-19.

The soluble specific substance extracted from Rickettsia and Proteus OX-19 is likely to be of a polysaccharide nature owing to the strong Molisch reactions obtained with such extracts, the heat stability and the negative protein reactions (biuret). Since, however, it still contains 7 per cent nitrogen, this is not certain. In the antigenic composition of both Proteus X-19 and typhus Rickettsia there is a common soluble specific factor which is responsible for the Weil-Felix reaction.

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