The hamster-adapted New-castle virus (California strain Number 11914) of the 203rd passage has been successfully transmitted to Swiss albino mice and carried through 20 serial passages in this species by intracerebral inoculation. The virus produced symptoms of irritability and malaise usually followed by paralysis, and often accompanied by a characteristic nervous jerking with labored breathing. Mice showing typical symptoms of central nervous system involvement did not recover. Positive Newcastle chicken serum neutralized the virus from the 15th mouse passage while normal chicken serum failed to neutralize the virus. The virus of the 15th mouse passage titred 10−3 in mice by intracerebral inoculation, The mouse-adapted virus proved pathogenic for Syrian hamsters upon intracerebral injection.