By means of a method previously described for demonstrating the growth-inhibitory and pneumococcidal power of normal serum-leucocyte mixtures, a study was made to determine the action of pneumococcus soluble substance on the growth of strains of pneumococci avirulent for rabbits and cats. It was found that the presence of a very small amount of the purified soluble substance of the homologous type markedly altered the conditions in the mixture so that even a small number of avirulent pneumococci were enabled to grow in the serum and leucocytes of animals which possess the power to destroy ordinarily such pneumococci in relatively large numbers. The action of the soluble substance was shown to be highly specific to type. A Type II substance assisted the growth of only Pneumococcus Type II, likewise a Type III substance, the growth of Pneumococcus Type III only.

Experiments involving broth filtrates of young pneumococcus cultures in place of the purified soluble substances gave similar results, thereby establishing further the identity of the purified substance with the precipitable substance originally described in the filtrates of pneumococcus cultures.

The effect of the soluble specific substance of pneumococcus in the serum-leucocyte mixtures is, therefore, to confer on the avirulent pneumococcus of the homologous type the growth capacity of a virulent organism.

The mechanism of the reaction is briefly discussed.

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