1. Type-specific S pneumococci may be transformed from one specific S type into other specific S types entirely by in vitro methods.

2. R forms of pneumococci, derived from S forms of one specific type, may be transformed into S forms of other specific types by the following in vitro procedure:—the growth of small inocula of R forms in media containing vaccines prepared from heterologous S cultures.

3. Transformation of type may be effected in this procedure by the use of small quantities of S vaccine,—quantities representing the bacteria from as little as 0.1 cc. of the original culture.

4. Transformation of type, as induced by this procedure, is most readily effected by employing anti-R serum in the culture medium. Transformation of type may be effected, however, in media which do not contain anti-R antibodies.

5. Previous findings on the thermal characteristics of the property of S vaccines responsible for transformation of type have been confirmed and extended.

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