Every positive Wassermann is accompanied by microscopic (or submicroscopic) aggregation, which is not an essential feature of the reaction; conversely, after every positive flocculation test, the washed precipitate will fix complement. An excess of antigen removes both flocculating and complement-fixing substances completely (>95 per cent). Heating the lipoid-reagin precipitate to 100° for 1 minute destroys the sensitizing film of reagin globulin; the avidity for complement disappears simultaneously.

Both the flocculating and complement-fixing properties of syphilitic serum are therefore determined by the same substance, a specifically altered fraction of the serum globulin, reagin. The Wassermann reaction is thus entirely analogous to complement fixation by any antigen-antibody complex. The same film of denatured serum globulin which sensitizes the antigen particles, whether red cells, bacteria, protein, or colloidal lipoid particles, to discharge and aggregation by electrolytes, also endows them with an avidity for complement.

The pathogenesis of reagin will be discussed in a forthcoming paper.