A single dose of 150 mg/g of cyclophosphamide (CY), given 3 days after intracerebral (i.c.) inoculation of lymphocytic choriomeningitis (LCM) virus, protected over 90% of adult BALB/c mice against acutely fatal choriomeningitis. Surviving mice became persistently infected carriers, with high virus titers in blood and brain. Immunofluorescent examination of the brain showed that in CY-induced carriers infection was initially confined to the choroid plexus, ependyma, and leptomeninges, but over the next 30 days gradually spread to the neural parenchyma, most notably to the molecular layer of the cerebellum. By contrast, LCM virus-carrier mice produced by neonatal virus injection and examined as adults, showed a much less marked infection of choroid plexus and much more widespread infection of parenchyma, with a different distribution among brain nuclei, including heavy infection of the Purkinje cells of the cerebellum.