When diphtheria toxin is injected in a suitable form and in sufficient quantity into an animal, antitoxin will presently appear in the blood. If the injection be made into an animal that has not previously received a stimulus there is a latent period of about three weeks before any antitoxin can be detected. The amount present in the blood gradually increases, reaching a maximum about eight weeks after the injection, and then a gradual fall in level of antitoxic content is seen. If, however, the same amount of the same antigen be injected into a previously treated animal, antitoxin appears in the circulation at a far greater rate. The latent period is only three days, and the maximum antitoxin level is reached in about eight days. The maximum level reached is from 10 to 100 times that reached after an injection into a normal animal. The two types of response are illustrated in Charts 1 and 2 and also in Chart 3, which shows the antitoxic content of a horse after two separate injections of a toxin antitoxin mixture. There is a great contrast between the extent and rapidity of the antitoxic response on the two occasions.