The amount of capsular polysaccharide formed in vitro by strains of pneumococcus types II, III, and VII of different virulence for mice has been measured by quantitative precipitin reactions. The quantity produced by strains of high virulence was greater for all three types than that formed by moderately virulent or avirulent strains.

It is suggested that an important factor concerned in variations of virulence of pneumococci for mice is the genetic apparatus that controls the amount of SSS synthesized by the cells.

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