By the aid of freezing and thawing, cell-free infectious virions were detected from an apparently nonproductive Vero cell line infected with Niigata- 1 strain of subacute sclerosing panencephalitis virus. The production of infectious virions was limited in amount and such virions were detectable only during a limited period after cell subculture. The infectious virions were filtrable through a 0.65μ membrane filter and neutralized completely by an antiserum against measles virus. The virions were banded at the density of 1.132, while Edmonston strain of measles virus banded at 1.164 in potassium tartrate density gradients. Infectious virions were also released from infected Vero cells by treatment of the cells in a hypotonic solution to an amount comparable to that obtained by freezing and thawing. Infection of normal culture of Vero cells with the infectious virions readily established a virus-cell interaction identical to that in the original infected culture from which the virions were recovered.