A study of pneumococci isolated from individuals suffering from lobar pneumonia has shown that the majority of these organisms fall into definite biological groups. These groups have been arbitrarily numbered from I to IV. The first three groups consist of organisms which within the group are closely related to each other by certain immunological reactions; i. e., protection and agglutination. Extensive study has failed to reveal crossing in either of these reactions between members of separate groups. The fourth group is formed of a series of independent varieties which cannot be definitely related to one another by the immune reactions employed. Up to the present time we have observed no tendency of these organisms to lose their specific characters, nor have we observed a change of one type into another. These groups vary in their pathogenicity for human beings, and in the order of their virulence are as follows: group III, group II, group I, group IV. The degree of protective power developed in the sera of animals immunized against members of the different groups varies inversely with the virulence and with the amount of capsular development. This, however, applies only to tests of passive immunity. The highly virulent groups give as good active immunity as those of lower virulence, if not better. In view of these constant differential characters of the pneumococcus, it was deemed advisable to study the pneumococci occurring in normal sputum. It has been commonly assumed that infection in pneumonia is autogenic, and occurs from the invasion of the lungs by a pneumococcus habitually carried in the mouth. If this is so, we should find the same types in the normal mouth as occur during the disease. Examination of a series of normal individuals showed this not to be the case. In no instance was an organism found which could be grouped with any of the fixed types of pneumococcus. All exhibited the same characters as those organisms obtained from lobar pneumonia which belong to group IV. Inasmuch as organisms belonging to this group are of low virulence, and are responsible in our experience for only 20 per cent. of the cases of pneumonia, it is at once manifest that the majority and more virulent cases of pneumonia are due to organisms which are not found in normal mouths. To gain further evidence of this difference, a study has been made of convalescents from pneumonia who had been infected by typical organisms. During the period of recovery these typical organisms are supplanted by the type which occurs in normal mouths. The period of disappearance of the typical varieties has varied. The shortest time in which disappearance has occurred has been twelve days, and the longest period in which typical organisms have been carried has been ninety days. In the latter instance the patient was lost sight of, so that he may well have carried the virulent form for a longer period of time. In general, when typical organisms persist for a long time, there is delay in the healing of the lung lesion. If recovery is prompt, as a rule the virulent types disappear rapidly. We have said that the virulent types do not occur in normal mouths. There are exceptions to this observation. In a number of instances organisms belonging to the typical groups have been isolated from the mouth sputum of healthy individuals. So far this has occurred only in individuals intimately in contact with cases of lobar pneumonia. Wherever typical organisms have been obtained under such circumstances, the type has always corresponded to that with which the case of pneumonia was infected. Such individuals, therefore, become infected with virulent types of pneumococcus by contact, and may be regarded as healthy carriers of disease-producing types. This study makes it probable that the majority of cases of pneumonia are dependent upon either direct or indirect contact with a previous case. Mere infection of the mouth by virulent types is by no means sufficient to cause the disease. In order to invade the lungs, these virulent types must find the circumstances favorable, or a suitable condition must arise during the period when they are harbored in the mouth. Comparative study of certain strains of pneumococci received from South Africa suggests that new groups of parasitic organisms develop only during the period of high racial susceptibility. A like condition of affairs is brought about when a group of hitherto unexposed individuals is brought into contact with an infectious microörganism. The development of racial immunity soon limits the number of new types which may arise. The suggestion is made that strictly parasitic races of microorganisms are pure lines and have established themselves as parasites during a period of high racial susceptibility.