1. When mice are infected intraperitoneally with Langat virus only a small proportion develop clinical encephalitis, but all mice have substantial titres of virus in the brain and also incontrovertible histological evidence of encephalitis.
2. When specific antibody is given intraperitoneally or intravenously to mice during the first 3 days after intraperitoneal infection with Langat virus, the viraemia (normally maximal during this period) is depressed, the production of antibody is depressed or delayed, and the incidence of clinical encephalomyelitis is increased significantly.
3. Specific antibody given intraperitoneally or intracerebrally before infection, protects the animals from encephalitis.
4. These findings are discussed in terms of the histology of the central nervous system of the affected mice.
We are very grateful to Miss S. J. Illavia, B.Sc., and to Miss G. E. Fairbairn for their skilled technical assistance; and to Mr S. Peto of the Microbiological Research Establishment for statistical advice.
This work was made possible by a generous grant from the Wellcome Trust and the Endowment Funds of St Thomas's Hospital