In the earlier days of hematology it was supposed that the leukocyte count was a quite reliable guide to the extent, progress and severity of various infections. It was shortly recognized, however, that other factors entered into the problem and that the degree of leukocytosis was more of an index of the amount of resistance, the fighting force possessed by the body, than of the virulence of the infection or of the actual progress of the combat between the invading bacteria and the natural resisting powers of the organism. Thus a normal leukocyte count or slight leukocytosis might mean little or no infection with no need of great resisting force or, on the contrary, it might be indicative of a very severe infection by which the body was overwhelmed without even a chance of combating it. Some other additional index was needed to complement this early one. Such an index