The biologic classification of pneumococci has been a great stimulus to the bacteriologic study of pneumonia and to efforts directed toward the perfection of an efficient serum treatment for this disease. This classification is a result of the work of several investigators, the chief of whom are Neufeld1 and Lister2 abroad, and Dochez and Gillespie3 in this country. The differentiation of pneumococci into types is based on their capacity to produce specific agglutinins, precipitins and protective bodies. The investigators at the Rockefeller Institute in their original studies found that there were three dominant biologic types of pneumococcus, each possessing specific and characteristic immune reactions. In their experience, the organisms of these three types comprised about 80 per cent of all strains encountered in patients with pneumonia, and represented apparently fixed types of a highly parasitic nature. These three types have been referred to as types I, II and III. In