1. Young cats inoculated intracerebrally with relatively large doses of neurotropic or viscerotropic virus, and intraperitoneally with visceropic virus fail to show circulating virus up to 12 days post-inoculation. 2. Such cats also fail to show symptoms referable to the virus. It was impossible to isolate virus from the brains of inoculated cats dying in the course of the experiments, and histological examinations of the brains failed to show escephalitis (only one case with toxic encephalitis). 4. The development of serological immunity after 30 days was observed in all but two of the cats inoculated intracerebrally or intraperitoneally with neurotropic virus; with Asibi strain, only two positive results were obtained, with the intracerebral route. 5. Young cats are relatively unsusceptible to yellow fever virus, only giving the reaction of immunity.