1. A correlation appears to exist between the failure of certain strains of Pneumococcus Type III to grow at 41°C. and their lack of virulence for rabbits.
2. It is likely that the capacity to grow at 41°C.—an attribute constantly but not exclusively associated with strains of Pneumococcus Type III virulent for rabbits—is a prerequisite, but not the sole factor, in determining their virulence for these animals.