Purified diphtheria antitoxic horse pseudoglobulin has been prepared which is homogeneous by sedimentation, diffusion, and electrophoresis. Immunologically, however, the preparation contains only 43.5 per cent antitoxin specifically precipitable by toxin. The inactive pseudoglobulin remaining after specific precipitation was found to have the same physical and chemical properties as the original antitoxic pseudoglobulin. Although the molecular weight of antitoxin is the same as that of the normal horse serum globulins, the electrophoretic mobility does differ from those normally present.

The molecular weight of diphtheria toxin is 70,000 and of antitoxin is 150,000.

From ultracentrifuge studies on the two reactants and on mixtures of toxin and antitoxin in the soluble inhibition zones, the average molecular composition of the specific floccules at certain reference points throughout the equivalence zone and the maximum "valence" of toxin and antitoxin with respect to each other have been calculated.

The significance of the results has been discussed in relation to antigen-antibody reactions in general and a possible explanation for the exceptional behavior of the toxin-antitoxin reaction in the region of excess antitoxin has been suggested.

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