In a study group of 40 children who had been admitted to hospital with acute encephalitis, the disease was due to infection with Japanese encephalitis virus (JEV). Three children developed recurrence of disease 8-9 months later. No virus had been isolated from these three patients during the acute stage of their illness, but virus was recovered from all during the recurrence phase by co-cultivation of their peripheral blood mononuclear cells in primary mouse embryo fibroblast cultures. Virus was also recovered by co-cultivation of peripheral blood mononuclear cells collected 8 months after their acute disease from three out of eight randomly selected asymptomatic children within the study group but not from similar cultures set up from JEV-seronegative children used as controls. Virus was also isolated by co-cultivation of T lymphocytes of asymptomatic children as detected by indirect immunofluorescence or by inoculation in mice.