Measles-like virus has been isolated from the brain cells of patients with subacute sclerosing panencephalitis (SSPE). The role of the host immune response in the causation of SSPE has been supported by several findings including the following: (1) progression of disease despite high levels of humoral antibody; (2) presence of a factor that blocks lymphocyte-mediated immunity to SSPE-measles virus in SSPE cerebrospinal fluid (CSF); (3) lysis of brain cells from SSPE patients by SSPE serum or CSF in the presence of complement (a similar mechanism could cause in vivo tissue injury).Higher-than-normal levels of serum antibodies (Ab) to measles virus and local synthesis of measles Ab in CSF, as evidenced by the oligoclonal IgG, imply a connection between the virus and multiple sclerosis (MS). However, the other studies have implicated the other viruses. Several studies of cell-mediated hypersensitivity to measles and other viruses in MS have been done, but the results have been conflicting. Definite conclusions regarding defects in cellular immunity in this disease cannot be reached until further research is completed.