Diphtheria antitoxin has been shown to be inactivated in the presence of urea. In 7.5 molar area at pH 7.82 nearly half the activity is destroyed in 24 hours, as measured by the neutralizing power of toxin. The reaction appears to be first order with respect to antitoxin, since the initial rate is independent of the antitoxin concentration, but the rate decreases rapidly with time. Higher or lower pH of the presence of M/10 sodium sulfite increases the antitoxin inactivation.

The meaning of the results is discussed.

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