Summary: The immunological properties of a polysaccharide prepared from type I meningococci and of antimeningococcal horse sera were studied by means of a quantitative analysis of the precipitative reaction. The results showed that the polysaccharide was a mixture of precipitinogens, and was therefore not immunologically pure, as was previously thought. The data indicated that the polysaccharide contained not only a type-specific component but also a considerable proportion of a group-specific one; that the monovalent type I serum which was studied contained only type-specific antibody; and that the polyvalent sera which were studied contained more group- than type I-specific antibody.