A simple quantitative technic has been described for the mechanical determination of the potency of antipneumococcal serum, types I and II, in terms of mouse-protective units. It consists essentially in titrating, by means of the photronreflectometer (1), known dilutions of the homologous polysaccharide with varying dilutions of serum to a galvanometric-reading corresponding to a dilution of SSS equivalent to a certain amount of a standard serum (see table 3). Similarly, milligrams per cubic centimeter of polysaccharide in an unknown solution may be determined by testing varying dilutions of the unknown with a serum on which the unit-potency has been established and titrating to equivalent unit and galvanometer-reading (table 3).

A rapid method for testing the purity of a preparation of polysaccharide and its suitability for use in the above photronreflectometric titrations is described. Essentially this method consists of: first, determining the equivalent combining proportions of serum and homologous polysaccharide (see (1)) in the zone of slight excess of antibody and then determining whether or not these combining proportions hold for halved dilutions of the serum and SSS (see (2)). If they do not hold, then the preparation contains other constituents than the homologous SSS and cannot be used in the photronreflectometric titrations described.

The chemical aspects of the data presented in this report will be discussed in a subsequent publication.

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