1. Quantitative data are given on the effect of variations in the time of contact and the proportions of the reactants on the quantity of complement combining component nitrogen (C'1 N) found in active guinea pig serum.

2. C'1 N was the same when determined with precipitates containing excess antibody or excess antigen.

3. Finely divided specific precipitates took up the complement combining component (C'1) from subsequently added guinea pig serum almost as well as specific precipitates formed in the presence of complement.

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