Three men who had worked at the same animal research facility and had had contact with macaque monkeys were infected with B virus (Herpesvims simiae). Their clinical presentations varied from self-limited aseptic meningitis syndrome to fulminant encephalomyelitis and death. Patient 1 was treated only after a respiratory arrest and other signs of advanced brain stem dysfunction had occurred. He died 8 days after hospital admission, despite treatment with acyclovir. Patient 2 presented with subtle signs and symptoms of brain stem encephalitis. He received antiviral therapy with intravenous ganciclovir. Patient 3 had a headache without meningismus and was also treated with acyclovir. Both patients 2 and 3 survived and did not have objective sequelae. Viral culturing, ELISA and western blot antibody testing, and magnetic resonance imaging all proved useful in the diagnosis of these patients' conditions.