A mouse tumor cell line (MCT) persistently infected with mumps virus (strain Urabe) was obtained. No antibody or chemicals were required for establishment or maintenance of the infected cell line (M-MCT). No difference was observed by light microscopy between MCT and M-MCT cells, except for cytoplasmic inclusion bodies in 10--20% of the M-MCT cells. Hemadsorption of chicken erythrocytes, which did not necessarily coincide with inclusion positive cells was shown by 10--20% of the M-MCT cells. Immunofluorescence staining revealed that almost all M-MCT cells contained mumps virus antigens, which were seen mainly in the cytoplasm as fine granules, dots, or large clumps. Infectious mumps virus was consistently detected in the culture fluid and the released virus showed some temperature sensitivity when assayed at 34 degrees C and 40 degrees C. M-MCT was resistant to superinfection with homologous virus and showed some resistance to heterologous viruses. Thirty one clones of M-MCT were isolated by the soft agar method. Some, but not all, clones had viral antigen and all those with antigen released virus into the culture medium. The growth rates of MCT and M-MCT cells as monolayer cultures in vitro were similar, but the transplantability of M-MCT cells in syngeneic C57BL/6 mice was lower than that of MCT cells.