1. Better results in the production of diphtheria antitoxin can be obtained with greater experience in the selection of the most suitable type of horses to be used. Young animals are usually to be preferred. Over one-half of all such horses can be made to yield 300-unit serum, while a third will yield 5oo-unit serum.
2. High-test horses require a shorter time to immunize and will yield a potent serum for a longer period than will low-test horses.
3. The period of usefulness of an antitoxin horse is short, and on an average endures only a few months.
4. A horse having attained a maximal antitoxic height begins to suffer a decline in antitoxin, which is usually as rapid as the ascent has been, and is unaffected by subsequent injections of diphtheria toxin.