1. It has been shown that the ability of virulent S. influenzae to kill mice is enhanced by suspending the organism in mucin.

2. The occurrence of a true infection characterized by in vivo multiplication of the organism has been established.

3. The necessity for using a pure inbred strain of mice for this type of study has been confirmed.

4. The increase in mucin virulence of a strain of H. influenzae by repeated passage through mice has been shown.

5. The usefulness of this method in passive immunization studies has been demonstrated.

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