1. Pneumococci, when freshly isolated from the body, are able to live and multiply when a small number of them are inoculated into a small amount of broth. If, however, the inoculations are made in large amounts of broth, many more bacteria must be inoculated in order that they may grow. 2. It requires much smaller numbers of pneumococci to start a growth on agar than are required to start a growth in broth. 3. This predilection for solid medium disappears when the bacteria are grown for some time outside the body. 4. This phenomenon is not dependent on differences in chemical composition between the two media employed or on the presence of more available oxygen in one case than in the other. 5. It is probably dependent entirely on physical differences in the two kinds of media, and bears some relation to the differences in possibilities for diffusion in the two media.