An RNA-dependent RNA polymerase activity has been found copurifying with measles virus infectivity and complement-fixing antigen in three Vero cell-grown variants of measles virus: the attenuated Edmonston B strain, the natural nonattenuated Edmonston strain, and a subacute sclerosing panencephalitis isolate, IP-3. Incubation of purified measles virions with immunoglobulin G derived from sera of monkeys hyperimmunized against measles specifically removes activity sedimenting in the density region of measles virions. The requirements of the reaction, which is RNase sensitive, are similar to those reported for other paramyxovirus-associated activities, including detergent, divalent cation, ribonucleoside triphosphates, and a reducing agent. The size classes of RNA synthesized correspond to those found in measles-infected cells, including 50, 35, and 16 to 20S. The product RNA of the Edmonston B virus-stimulated reaction was rendered RNase resistant by annealing with RNA extracted from purified Edmonston B virions. RNA from uninfected Vero cells was ineffective in the annealing reaction. Measles virus has been classified as a paramyxovirus on the basis of morphology and RNA size (8, 19, 23, 24). Reports in the literature of RNA-dependent RNA polymerase in the virions of other paramyxoviruses (4, 11, 18, 22) suggested that measles virus might contain a similar activity. Here we describe and characterize a measles virus-associated RNA-dependent RNA polymerase activity. MATERIALS AND METHODS Cells and viruses. Vero cells, a continuous line of African green monkey kidney cells, were supplied by the Cell Biology Section, Bureau of Biologics, and were used both for virus production and infectivity assays (2). They were grown and maintained in Eagle minimal essential medium supplemented with 10% fetal calf serum, 0.03% glutamine, 50 ,ug of gentamicin per ml, and 0.4 pLg of amphotericin B per ml.