Technics have been found for intracerebral neutralization tests and intra-abdominal protective tests with the Semliki Forest virus. By means of these immunological tests the Semliki Forest virus has been differentiated from the viruses of yellow fever, Bwamba fever, St. Louis and Japanese B encephalitis, eastern and western equine encephalomyelitis, and from the West Nile virus. A limited immunity survey among humans residing in 4 widely separated areas of Uganda was made. It revealed that individuals residing in each of the areas sampled possess protective antibody against the Semliki Forest virus. The incidence of humoral antibody in the 4 areas, and among different age groups within them, was sufficiently variable to emphasize the specificity of the tests and to point to recent activity of the virus in widely separated areas having no contact. Sera from 20 species of African wild animals have been examined for the presence of antibody. Humoral immunity was present in individuals of 6 different species—all of them primates. The high incidence of humoral immunity among Cercopithecus nictitans, together with its relatively high susceptibility, point to the possibility that in the wild state this species may be frequently attacked by the virus. The studies reported in this and the preceding paper indicate that the Semliki Forest virus is probably an entity hitherto unknown.