[Defective interfering mumps virus produced by chronically infected cell cultures].

HEp-2 and L-41 culture chronically infected with mumps virus at late passage levels produced 10(2)--10(3) PFU/ml small-plaque slow-replicating virus and defective interfering particles (DIP) in significant amounts. The DIP were characterized by the polypeptide composition similar to that of the virus, the presence of subgenome sise of RNA, and marked interfering activity with regard to the original mumps virus.