The hydrogen ion concentration of minimum solubility of the water-insoluble protein of antipneumococcus horse serum soluble at various percentages of saturation with ammonium sulfate, namely 30, 36, and 50 per cent, was found to be as follows: for the 30 per cent, a zone between pH 5.8 and 5.9; for the 36 per cent, pH 6.1 to 6.2; and for the 50 per cent, pH 6.8 to 7.0. These same fractions of the protein obtained from normal horse serum all lay in the zone between pH 6.0 and 6.2. At the hydrogen ion concentration of minimum solubility, the protein of immune serum was from one-fifth to one-tenth as soluble as that obtained from normal serum. The protein obtained by dissociating the S. S.-antibody complex was found to have a hydrogen ion concentration of minimum solubility at pH 6.7 or above. The highest protective potency in relation to the amount of protein was found in the fractions precipitated by ammonium sulfate at 36 and 50 per cent saturation. The percentage of total serum protein (pooled serum) present in the different fractions was respectively, with 30 per cent, type I 4.6 per cent; normal 3.8 per cent; with 36 per cent, type I 7.3 per cent, normal 3.3 per cent; and with 50 per cent, type I 3.0 per cent, normal 0.9 per cent.

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