Cell-free preparations of “toxin” were made from two strains of , one known to be of high virulence and the other of low virulence in the mammary gland of cattle. These preparations were similar in their in-vitro properties and in their clinical and histopathological effect when inoculated into the mammary gland of mice. However, when live staphylococci were inoculated into the mammary gland of mice a difference in virulence corresponding to that seen in cattle was detected by the clinical response, and by the histopathological appearance, the number of organisms, and the amount of α-lysin present in the infected glands. The reasons for the failure of the low-virulence strain to express its potential virulence are discussed.


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