1. Guinea pigs infected with naturally pathogenic hemolytic streptococci (group C—Lancefield) develop a low grade chronic type of disease characterized chiefly by purulent lymphadenitis.

2. Cutaneous hyperreactivity to a crude streptococcal extract invariably occurred during the course of this infection.

3. Production of antibodies (precipitins and agglutinins) was studied.

4. The hemolytic streptococcal extract had a specific toxic effect, when tested in vitro, on cells from infected animals; this was shown by microscopic evidence of cellular injury, and by quantitative inhibition of cellular migration and growth. The specificity of the reaction was proven by testing with other cytotoxic substances.

5. There was no parallelism between skin hypersensitivity and humoral antibody titer.

6. There was no correlation between the degree of skin reactivity to the bacterial extract and the degree of sensitivity of splenic cells to the toxic action of the same extract in vitro.

7. Comparison of cellular sensitivity to tuberculin with cellular sensitivity to streptococcal extract in cultures of guinea pig tissues showed that the former was more intense and was more persistent on prolonged growth in vitro.

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