1. Type-specific S pneumococci may be transformed from one specific S type into other specific S types through the intermediate stage of the R form.

2. R forms of pneumococi, derived from any specific S type, may be transformed into S organisms of other specific types by the following procedure:—The subcutaneous injection, in white mice, of small amounts of living R forms together with vaccines of heterologous S cultures.

(i) S vaccines heated for 15' at temperatures between 60° and 80°C., are effective in causing R forms, derived from heterologous S types, to revert to the type of the vaccine.

(ii) S vaccines heated for 15' at temperatures between 80° and 100°C., are not effective in causing R forms, derived from heterologous S types, to revert to the type of the vaccine.

(iii) S vaccines heated for 15' at temperatures between 80° and 100°C., may cause 2 R and 3 R cultures to revert to their original S type.

(iv) S vaccines of any type, including Type I, heated for 15' at temperatures between 80° and 100°C., are not effective in causing 1 R cultures to revert to their original S type.

(v) S vaccines heated for periods as long as two hours at 60°C. are effective in causing R forms, derived from heterologous types, to revert to the type of the vaccine.

3. A single cell R strain, derived from a Type II S pneumococcus, has been successively transformed into a Type III S, a Type I S and a Group IV S culture.

4. Corresponding with the various degrees of "degradation" of the R form there are varying degrees of "development" of the S form.

5. The nature of the conditions responsible for alteration of type as induced by these procedures has been investigated and the causes responsible for the transformations are discussed.

6. All attempts to produce transformation of type in vitro have been unsuccessful.

7. The rô1e which the phenomenon of transformation of type may play in problems of infection and epidemiology is indicated.

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