1. The quantitative method for the estimation of agglutinins has been applied to antimeningococcal horse, rabbit, and chicken sera and to the sera of humans convalescing from meningococcus meningitis. The type-specific and group-specific agglutinin N can be measured, using homologous and heterologous suspensions of meningococci.
2. Type I horse, rabbit, and chicken antimeningococcal sera contain considerable amounts of antibody which cannot be removed either by Type II meningococcus suspension or by preparations of the Type I specific polysaccharide. This residual type-specific antibody has marked potency in protecting mice against subsequent infection with meningococci.
3. Most human convalescent sera contain group-specific antibody. Small amounts of protective antibody and of antipolysaccharide are also formed.
4. Type I antisera absorbed with Type I polysaccharide and with Type II meningococci could be used as a guide in the purification of this new antigen.