When rabbits are injected intravenously with a quantity of virulent streptococci or pneumococci sufficient to cause death within two to four days the septicemia takes a definite course with slight variations. The bacteria rapidly decrease in number from the time of the injection to from two to four hours, at which time the blood is sterile or contains only a few bacteria. Within five to six hours the bacteria reappear in the blood and steadily increase until the death of the animal. If the bacteria are less virulent, the same quantity of culture causes a chronic type of infection. The same initial decrease in the number of bacteria occurs. The reëntrance into the blood is somewhat delayed, the septicemia does not reach the height obtained in the acute cases, and a second fall occurs within the course of a few hours. These rabbits show a low blood invasion or a sterile blood culture for several days. During this time they become emaciated to a marked degree. Then the low septicemia rapidly rises or the rabbit with a sterile culture develops a severe septicemia within a few hours and death takes place from a few hours to two days thereafter. In this type of infection local lesions, pericarditis, pleurisy, peritonitis, etc., are usually found. In the infections which run an acute course no gross lesions are found. If the bacteria are still less virulent they never reënter the blood after the initial disappearance and the rabbits remain in good condition. In order to obtain uniform results, the quantity of bacteria injected must not be so large that the bacterial substances See PDF for Structure carried in are sufficient to cause an intoxication of the animal. If the quantity of bacteria injected is below this point the course of the infection depends largely upon the virulence of the infecting organisms. Yet variations in the natural resistance of individual animals may be sufficient to cause quite marked irregularities in the course of the infection. Pneumococci can be standardized so as to produce a particular type of infection more easily than streptococci. In general infections such as those produced by streptococci and pneumococci the number of the bacteria present in the circulating blood at a given time supplies accurate and delicate information regarding the severity of the disease. When the object is to determine the degree of virulence of bacteria, or of the efficiency of an experimental therapeutic method, the mere physical condition and mere death of the inoculated animals are not sufficient and satisfactory guides to the desired information. The death of the inoculated animal and the recovery of the infecting bacteria at autopsy do not give complete information concerning the intensity and course of the infection occurring during life. A large number of bacteria found in the blood and tissues at autopsy do not necessarily prove the existence of a heavy infection before the onset of the death agony, since it is a well known fact that bacteria multiply with enormous rapidity, once the natural resistance of the animal has been overcome. Therefore, if merely the life and death of the animal and autopsy findings must serve as our only guides, we shall lose much incidental information, perhaps of fundamental value. This may be especially true as regards the search for curative substances. Again, the individual animals of the same species, age, and apparently of identical physical condition react to the aggressive force of the infecting organisms variously. This fact is readily found out by the injection of a series of rabbits with lethal quantities of bacteria per body-weight, and by making tests at various periods before death results, which, in the case of streptococci, ranges from one to six days. Consequently a method which enables the determination of the degree and progress of the infection at any desired period is of obvious advantage.