1. The infectious agent of Colorado tick fever filtered through 181 mµ membranes caused infection in hamsters and one volunteer.
2. In five out of seven instances, hamsters were infected initially with serum filtered through 24 mµ membranes or the tick fever appeared on serial transfer.
3. We could not infect two volunteers with 24 mµ filtrates, though these filtrates did cause infection in hamsters as shown by development of the fever on serial transfer. In one instance, the serum of the hamsters rendered ill in this way was injected into the volunteer who had remained well after inoculation with the original material and it caused Colorado tick fever.
4. Normal hamster serum was passaged through 10 groups of animals and then two human volunteers were injected with it. They did not come down with Colorado tick fever, although both of them were susceptible to the disease as shown by the fact that they fell ill of it on later inoculation with serum from a natural instance of the disease.
5. The infectious agent of Colorado tick fever passes through 24 mµ membranes. On this basis, it is classified as a virus.