1. “Antigens” of varying sensitivity may be prepared by coating a dispersion of gum benzoin with saponin.2. Using suitable dispersions so prepared a zone of precipitation is observed which is more extended for specific than for normal sera. Differentiation is more pronounced as the region of the isoelectric point of the globulin is approached.3. Precipitation is effected by the ions of salts present in the solution discharging the negative antigen suspension which has been sensitised by the protein of the serum.4. The mechanism of sensitisation is described.5. Both albumins and globulins can protect or sensitise gum benzoin suspensions, the effect depending on an appropriate dilution. This effect occurs qualitatively irrespective of the hydrogen ion concentration of the medium. Quantitatively the position of the zone of precipitation and its extent is contingent on the reaction of the medium.6. The euglobulin fractions of normal and of luetie sera deprived as far as possible of their lipoids can be differentiated both by the Wassermann test, and by precipitation tests.7. Delipoided euglobulins from horse serum will yield a Wassermann positive reaction when used in certain concentrations. In more concentrated solutions an anticomplementary effect is exhibited.8. Evidence is presented for the view that the euglobulins of specific sera differ rather in their composition or state of aggregation than in quantity from the euglobulin of non-specific sera.9. The changes in sensitising power and in the Wassermann reactivity of euglobulin caused by repeated freezing or exposure to ultra-violet light run a parallel course.10. Attempts to change the Wassermann properties of normal sera are briefly described.