1. Single cell cultures of "R" pneumococci may revert in all respects to the "S" type. 2. In all instances in which reversion has occurred the "R" forms have invariably reverted to the same specific type from which they were originally derived. 3. The transformation "R" to "S" may be effected in vitro by growth in anti-"R" sera, as well as in vivo by the method of animal passage. 4. Reversion from "R" to "S" whether effected in vivo or in vitro, is always accompanied by the acquisition of maximal virulence.