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.