1. The symptoms, history of tick bite, characteristic fever curve, and white blood cell picture should enable the physician to make a diagnosis of Colorado tick fever in nearly every case. 2. The typical white blood cell picture is a depression of the total leucocytes with a shift to the left of the granulocytes. Basophilic cytoplasmic bodies appear occasionally in lymphocytes 3 to 4 days after clinical recovery. 3. The disease can be transmitted serially in human beings by parenteral injection of blood or serum. Such transfers have not resulted in decreased or increased virulence. 4. The naturally acquired and experimental cases of Colorado tick fever are identical in their manifestations. 5. An attack of Colorado tick fever confers a degree of definite immunity to the disease. 6. Colorado tick fever is not a mild form of Rocky Mountain spotted fever since individuals immunized with ground tick vaccine against Rocky Mountain spotted fever are still susceptible to Colorado tick fever. 7. Adult Dermacentor andersoni ticks allowed to feed on typical cases, then carried through to a new generation and fed on susceptible adults, failed to transmit the disease. 8. Colorado tick fever has been successfully transmitted to an experimental animal, the golden hamster.