Abstract
1. Considerable variation in the resistance of different rats toward Type II pneumococcus has been demonstrated. In general, older rats survive much greater doses than young ones, illustrating the acquisition of a natural partial immunity. The same is true for Type III but the immunity appears somewhat later in life and does not reach the same height. 2. An active immunity can be created against Types II and III in rats by feeding the dead organisms or the Berkefeld filtrate of the bile salt-dissolved cells. This immunity resembles that obtained against Type I in several respects.