Dose, as a prophylactic, 500 units; as a curative agent, 2,000-4,000 units or more.

A unit is the quantity of antitoxine necessary to prevent the death of a guinea-pig weighing 250 grammes when injected with 100 lethal doses of diphtheria toxin. The serum is the blood-serum of horses immunised by the injection at stated intervals with diphtheria toxin in amounts at first sublethal but finally many times the lethal dose. The blood is drawn from the horse under the most careful aseptic precautions, is allowed to clot, the serum removed and set aside for several weeks during which time a precipitate forms which is filtered off. The serum is now put up in suitable containers, usually with the addition of some antiseptic, such as phenol or cresol. Its antitoxic power is tested previous to its being placed in the containers and into each of these latter is put a definite number of units. The number of units and the date of the preparation of the serum must be placed on a label upon each container. The serum decreases in activity with age, losing 10 - 30% per annum. A dried serum is also prepared.