Animal inoculation has been referred to (1) as a method of assisting in the preparation of pure cultures of pathogenic organisms; (2) as a means of testing the poisonous properties of substances produced in bacterial cultures; (3) in order to test the ability of an organism to cause a disease; (4) for the production of various antibodies; it may be added (5) that some bacteria produce in the smaller experimental animals lesions which do not occur in animals naturally infected, but which nevertheless are characteristic for the given organism. The best illustration is the testicular reaction of young male guinea-pigs to intraperitoneal injections of glanders bacilli. Experimental animals are also inoculated (6) to test the potency of various bacterial and other biological products, as toxins, antitoxins, etc. Guinea-pigs are the most widely used experimental animals because they are easily kept and are susceptible to so many diseases on artificial inoculation. Rabbits are used very largely also, as are white mice. For special purposes white rats, pigeons, goats and swine are necessary. For commercial products horses (antitoxins) and cattle (smallpox vaccine) are employed. In the study of many human diseases the higher monkeys and even the anthropoid apes are necessary, since none of the lower animals are susceptible. The commonest method of animal inoculation is undoubtedly the _subcutaneous_. This is accomplished most readily with the hypodermic needle. The skin at the point selected (usually in guinea-pigs the lateral posterior half of the abdominal surface, in mice the back near the root of the tail) is pinched up to avoid entering the muscles and the needle quickly inserted. Clipping the hairs and washing with an antiseptic solution should precede the inoculation as routine practice. Frequently a small “skin pocket” is all that is needed. The hair is clipped off, the skin pinched up with small forceps and a slight snip with sharp scissors is made. The material may be inserted into this pocket with a heavy platinum needle. _Cutaneous_ inoculation is made by shaving the skin and rubbing the material onto the shaved surface or scratching with a scalpel or special scarifier, but without drawing blood, and then rubbing in the material to be inoculated. _Intravenous_ injections are made with larger animals. In rabbits the posterior external auricular is a convenient vein. In larger animals the external jugular is used.