Eight individuals who had recovered from Weil's disease from two to over 20 years still had agglutinins for type I Leptospira icterohaemorrhagiae present in their circulating blood in significantly high titer. The average level of agglutinative titer during the active phase of the disease in seven cases was 1:23,000—. The minimal dilution in which agglutination occurred between two and 20 years after complete recovery from the disease was 1:100, while the maximal was over 1:30,000.

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