Change of non-typable (R) strains of H. influenzae into the specific type of origin or new types (S) has been effected in vitro in a predictable fashion within a single 24 hour growth period, by a desoxyribonucleic acid-containing fraction isolated from type-specific cells of the type desired.

Only a small proportion of the population of R cells are susceptible to the change induced in inherited characteristics by the desoxyribonucleic acid fraction. The data suggest that the number of susceptible cells present in any given population size varies with the specific type of origin of the R cells; a lesser degree of variation in different independent cultures of the same strain and population size has been demonstrated. The results suggest that the R H. influenzae cells which are susceptible to transformation arise as the result of mutation.

It has been demonstrated that the reaction necessary for transformation takes place virtually immediately if susceptible cells are present. Furthermore it has been shown that the change which enables an R cell to form a colony of type-specific organisms has been completed within 15 minutes in an environment which does not permit cell multiplication.

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