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.