Serological evidence suggests that approximately 12% of rhesus monkeys recently imported into the United Kingdom have been infected with B virus. The virus persists as a latent infection and has been recovered from about 40% of trigeminal ganglia taken from seropositive monkeys. Attempts to demonstrate the excretion of virus by such animals, either spontaneously or in response to procedures likely to induce reactivation of the latent virus, have so far been unsuccessful. However, it is likely that virus is excreted from seropositive animals, albeit rarely, and that they constitute a hazard to laboratory workers. All laboratory-held monkeys should be screened for the presence of antibodies and, ideally, the positive reactors killed. A number of agents are being tested for possible prophylactic and therapeutic use in human cases of B virus encephalitis. Preliminary results indicate that vidarabine has some slight activity, but the infiltration of the infected wound with immune serum appears to be a better form of immediate prophylaxis.