Potent monovalent botulinus antitoxic serums, types A and B, were produced by immunization of horses with homologous botulinus toxins. The maximum titer of 1400 units per cubic centimeter was obtained with type A toxin.

Not all horses respond to such rapid increases in the doses of botulinus toxin as were given in the first method of immunization described. Two of the three horses immunized by this method succumbed after the tenth or twelfth dose with typical symptoms of botulism.

Antitoxin production with the one horse immunized by the second method (slow increase of dosage with intervals of rest) was slower than with the horses immunized by the other methods.

The small doses of toxin given at the beginning in the third method of immunization (frequent injections of toxin with slow increase in dosage) apparently stimulated antitoxin production as readily as the larger doses used at the beginning in the first method (frequent injections of toxin with rapid increase in dosage) and with less danger to the animal.

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