Neutralization of diphtheric toxin by antitoxin is inhibited after the exposure of a mixture of antitoxin and methylene blue to light. Flocculation is also inhibited. In our experiments no attempt was made to demonstrate complete inactivation, but judging by the data this should not be difficult. Inactivation does not occur when the mixture of antitoxin and dye is kept in an evacuated vessel, nor when air is admitted and light excluded.

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This work was made possible because of a grant given by Lederle Laboratories, Inc., Pearl River, N. Y., which the author gratefully acknowledges.

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