The immune bodies of antipneumococcus serum are completely precipitated by 38 to 42 per cent. saturation with ammonium sulphate. They are incompletely precipitated by (a) ammonium sulphate in less than 38 per cent. saturation, (b) saturation with sodium chloride, (c) dilution and saturation with carbon dioxide, (d) removal of crystalloids by dialysis. The immune bodies of antipneumococcus serum are, therefore, associated or combined with that fraction of the globulins precipitated by 38 to 42 per cent. saturation with ammonium sulphate. The immune body fraction does not correspond exactly with the ordinary euglobulin (one-third saturation with ammonium sulphate or complete saturation with sodium chloride) or with the insoluble globulins precipitated by carbon dioxide or dialysis. These fractions carry with them only a part of the immune bodies. Neither the albumin nor that fraction of the globulin not precipitated by 38 to 42 per cent. saturation of ammonium sulphate contain any of the demonstrable antibodies. The most promising method for the practical purification of the immune bodies occurring in antipneumococcus serum appears to be precipitation by 38 to 42 per cent. saturation with ammonium sulphate.