Virologic factors in the pathogenesis of subacute and chronic measles encephalitis are being investigated. Strains of measles virus and cell cultures containing defective measles virus genome have been isolated in our laboratory from brains of patients with subacute sclerosing panencephalitis (SSPE). These viruses and cells are being characterized by a combined biochemical-immunologic-genetic approach. Attempts are being made to detect virus or viral products in brains of patients with subacute and chronic nervous system diseases other than SSPE and multiple sclerosis.