Three cases of Rift Valley fever in human individuals are reported. The virus was recovered from the respiratory tract of the patients and was transmitted to ferrets by the intranasal route. The experimental disease so produced in ferrets is characterized by fever, marked pulmonary lesions, and hemorrhagic phenomena.

The results indicate that the virus of Rift Valley fever belongs to the group of filterable viruses which may gain entrance to the human body through the respiratory tract.

The differential diagnosis of Rift Valley fever and influenza is discussed. While, clinically, this is a difficult problem, the diagnosis may be readily established through animal experimentation.

Certain observations concerning the influence of the route of administration on the protective action of immune serum in serum-virus mixtures are presented.

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