Intracerebral inoculation of twelve spider monkeys with large doses of a virulent strain of Japanese encephalitis virus produced a subclinical encephalomyelitis. When an immunosuppressive dosage schedule of cyclophosphamide was given to a group of four monkeys concurrently with virus, all animals developed prostrating paralysis 12–14 days after infection. Virus was isolated more regularly from the blood and throat swabs of animals treated with cyclophosphamide than from controls. Monkeys inoculated with virus only developed serum antibody, but no antibody was detected in suppressed animals. At time of killing, all spinal cords from suppressed monkeys yielded virus and presented a histological picture resembling that of fatal poliomyelitis, but with a markedly reduced inflammatory response. Virus was isolated from only one of three cords from animals inoculated with virus alone, and histological examination indicated less severe neuronal destruction.