Reference to the table of blood reactions given on p. 71 will show that in order to discover the blood group of any individual it is only necessary to test his corpuscles against the serum of Groups II and III. These reactions may be recapitulated as follows: (i) If he be Group I, his corpuscles will be agglutinated by the serum of Groups II and III. (ii) If he be Group II, his corpuscles will be agglutinated by the serum of Group III only. (iii) If he be Group III, his corpuscles will be agglutinated by the serum of Group II only. (iv) If he be Group IV, his corpuscles will be agglutinated by neither serum. Only the serum, therefore, collected from people known to belong to Groups II and III need be kept in stock. This can generally be obtained from the Lister Institute, and if kept sterile will retain its agglutinating properties for some months, but under no circumstances should serum more than six months old be used, since the consequences of a failure to agglutinate may be very serious. Nevertheless, the agglutinins contained in serum are very resistant to physical and chemical changes in their environment. Dried serum has been successfully used for testing purposes, and Culpepper has shown that the reactions are not interfered with by cold or by heat until actual coagulation of the serum takes place. Bacterial contamination does not affect the reactions, so that the serum is still active even when putrid. Various methods have been used for preserving the serum. Its properties are not affected by the addition of dilute cresol (1 : 250) or of chloroform. In the absence of any stock sera, the agglutinating test may be applied directly. A few cubic centimetres of blood are taken from the patient, and the serum as soon as it has separated is tested against the corpuscles of the prospective donor. If agglutination occurs, this donor is at once excluded. If no agglutination occurs, he is either of the same group as the patient or belongs to a compatible group. Supposing that a donor actually of the same group as the patient is wanted, then the reverse test must be performed in addition, that is to say, the corpuscles of the patient must be tested against the serum of the donor. If both tests are negative, then donor and patient are proved to be of the same group. The method of direct test cannot be applied in an emergency owing to the loss of time involved; it is better, therefore, that anyone who intends to be ready to perform a blood transfusion should always have serum of Groups II and III immediately available.