SUMMARY: A quantitative method for the study of the specific agglutination of micro-organisms is described. The degree of agglutination is estimated from the sedimentation curves of suspensions to which varying amounts of serum are added, which are compared with that of a control suspension without serum. The time of centrifugation required for sedimentation of 50% of the organisms in the serumsuspension mixtures related to the corresponding time for the control suspension gives the relative 50 % sedimentation times which, within a certain region, show a linear relation with the amounts of serum used. The activity of sera is expressed by their agglutinin content and by their agglutinating capacity, given by the slope of the time-serum regression line.

Rickettsias of the typhus group and their corresponding sera are clearly distinguished by the method described. Proteus OX19 agglutinating sera of different origins (pregnant women, murine typhus and Proteus-immune rabbit), with the same agglutinating titre by the dilution method, are distinguished according to their origin by the quantitative method. The application of the method to the study of other groups of micro-organisms is suggested.


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