I began with a very interesting fact discovered by my fellow-worker, Dr. Römer. He showed that true albumins penetrate unchanged the intestinal mucous membrane of new-born foals, calves, and smaller laboratory animals, and that they produce the same action on the organism as when they are injected directly into the circulation. In adult animals of all species, on the contrary, the true albumins must first be digested into peptones before they can pass through the mucous membrane. The anti-diphtheria serum and the anti-tetanus serum contain curative substances in the form of true albumins. If such a serum be introduced into the stomach of a healthy, full-grown animal or man, not a trace of these bodies passes into the blood. On introducing the serum, however, into the stomach of the new-born, the unchanged antitoxic albumin can almost entirely be demonstrated in the blood. This discovery indicates that the mucous membrane of adults, acting as a dialyzing membrane, does not allow the large molecules of true albumins to pass through unchanged, whereas the mucous membrane of sucklings behaves more like a very porous filter. It was but a step from this discovery to the assumption that the mucous membrane of infants might behave similarly toward bacteria. For my first experiments I selected anthrax bacilli, which, when free from spores and given in milk per stomach, do not affect adult guinea pigs at all. They are quite rapidly thrown off with the excreta, remaining, however, a little longer in the cæcum. When the same dose of bacilli was administered in this way to guinea-pigs less than eight days old, they died just as rapidly of anthrax as by the customary method of infection. Next I tried anthrax bacilli whose virulence had been reduced. These are harmless when injected subcutaneously into guinea-pigs. _After feeding these weakened bacilli to new-born guinea-pigs, the blood of the animals contained anthrax bacilli, though the animals did not die. Incidentally, a fact of considerable theoretical importance was discovered: that the anthrax bacilli possess a very intimate affinity for the endothelium of the heart and blood vessels._