The method of preparing antitoxins is essentially the same in all cases, though differing in minor details. For commercial purposes large animals are selected, usually horses, so that the yield of serum may be large. The animals must, of course, be vigorous, free from all infectious disease. The first injection given is either a relatively small amount of a solution of toxin or of a mixture of toxin and antitoxin. The animal shows more or less reaction, increased temperature, pulse and respiration and frequently an edema at the point of injection, unless this is made intravenously. After several days to a week or more, when the animal has recovered from the first injection, a second stronger dose is given, usually with less reaction. Increasingly large doses are given at proper intervals until the animal may take several hundred times the amount which would have been fatal if given at first. The process of immunizing a horse for diphtheria or tetanus toxin usually takes several months. Variations in time and in yield of antitoxin are individual and not predictable in any given case. After several injections a few hundred cubic centimeters of blood are withdrawn from the jugular vein and serum from this is tested for the amount of antitoxin it contains. When the amount is found sufficiently large (250 “units” at least for diphtheria per cc.)[24] then the maximum amount of blood is collected from the jugular with sterile trocar and cannula. The serum from this blood with the addition of an antiseptic (0.5 per cent. phenol, tricresol, etc.) constitutes “antidiphtheritic serum” or “antitetanic serum,” etc. All sera which are put on the market must conform to definite standards of strength expressed in “units” as determined by the U. S. Hygienic Laboratory. In reality a “unit” of diphtheria antitoxin in the United States is an amount equivalent to 1 cc. of a given solution of a _standard_ diphtheria _antitoxin_ which is kept at the above-mentioned laboratory. This statement, of course, gives no definite idea as to the amount of antitoxin actually in a “unit.” Specifically stated, a “unit” of antitoxin contains approximately the amount which would protect a 250 gram guinea-pig from 100 minimum lethal doses of diphtheria toxin, or protect 100 guinea-pigs weighing 250 grams each from one minimum lethal dose each. The minimum lethal dose (M. L. D.) of diphtheria toxin is the least amount that will kill a guinea-pig of the size mentioned within four days. Since toxins on standing change into toxoids to a great extent, the amount, of antitoxin in a “unit,” though protecting against 100 M. L. D., in reality would protect against about 200 M. L. D. of toxin containing no toxoid.