During bacterial infections, the intensity of the polymorphonuclear leukocytosis depends on the bacterium but also on the mechanism and extent of the infection. Polymorphonuclear leukocytosis is greater during pyogenic and anaerobic infections. It is due to deep suppuration, septicemia of thrombophlebitic origin, acute endocarditis, purulent meningitis and pneumonia. The increase in the number of polymorphonuclear cells is, on the other hand, less marked in sub-acute bacterial endocarditis. Apart from bacterial infections, a polymorphonuclear leukocytosis is common in inflammatory disease, such as tissue necrosis and several malignant diseases. It may also be due to drug allergy.