An infectious, cytopathic paramyxovirus was rescued from the peripheral blood leukocytes of a patient with subacute sclerosing panencephalitis (SSPE) by cocultivation procedures. Cells infected with the virus (1) demonstrated polykaryocytosis, (2) contained cytoplasmic inclusion bodies of paramyxoviral nucleocapsids, (3) hemadsorbed monkey erythrocytes, and (4) reacted with fluorescein-conjugated antisera to measles virus. On the basis of these observations, the virus was initially believed to be measles-like virus similar to those that have been rescued from other patients with SSPE. However, subsequent immunologic and molecular studies showed that the virus was distinct from measles virus and was more closely related to simian virus 5. These findings suggest that paramyxoviruses other than measles virus can be nonproductively carried by patients with SSPE and should not be designated as "measles like" or "measles related" solely on the basis of their biologic characteristics.