In diphtheria toxins purified by a considerable variety of methods, the number of m.l.d. per Lf unit generally falls between 20 and 40 provided the toxin has not been altered by the purification process used.

The Kf of toxin is a definite exponential function of the Lf concentration: Kf = k/Lf1.2 where (k) is a constant characteristic of the toxin and antitoxin tested.

Concentrated bacterial protein may modify or obscure the flocculation reaction.

The increase in Kf produced by acid, alkali and acetone, appears to be associated with denaturation of the toxic protein. This change is not the same as modification of toxin to toxoid.

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