(1) The originally distributed ECHO-13 prototype virus, Hamphill 2-188-20 passage 7, is a mixture of ECHO virus types 1 and 13 ECHO-1 was found in higher titer than ECHO-13 during passage and only the former was isolated at terminal dilution. It was necessary to neutralize ECHO-1 virus with antiserum to demonstrate the presence of ECHO-13 in this mixture and this apparently did not alter its antigenic properties. ECHO-13 virus was successfully reisolated in pure form from the original rectal swab elute after 4 years of storage. (2) Plaque purified strains of reisolated 2-188-20 virus and Del Carmen 11-4-1 virus isolate were examined as possible ECHO-13 prototypes. The plaque purified 11-4-1 strain produced a broader spectrum antiserum in monkeys in early test bleedings and produced a higher infectivity titer in monkey kidney cell culture. It has been accepted as the prototype for ECHO-13 by the Committee on Enteroviruses. (3) The biologic properties of ECHO-13 virus in certain animals, chick embryos, cell culture and cell culture fluids are described. A clinically inapparent meningoencephalitis occurred in inoculated monkeys.