AbstractA persistent infection of baby hamster kidney‐21 (BHK‐21) cells with mumps virus (BHKpi) was maintained for over 60 cell passages in the absence of antiserum. Viral persistence was demonstrated in the cultures by hemadsorption, immunofluorescence, multinucleate syncytia, and released mumps virus at the level of 102–103fluorescent focus‐forming units/ml. No detectable levels of interferon were found in cultures persistently infected with mumps virus. Approximately 85–95% of the cells contained viral antigens. Nuclear fluorescence was observed in the persistently infected cells. Mumps virus from persistently infected cultures (MuVpi) was more heat‐labile than wild‐type mumps (MuVo) when subjected to 40°C. BHKpi cells had a more rapid doubling time and a higher cloning efficiency in soft agar in comparison to BHK‐21 cells. MuVpi was also found to be temperature‐sensitive. The temperature‐sensitivity of MuVpi was determined by the efficiency of plating at 33° and 39°C. MuVpi readily established a persistent infection in BHK‐21 cells with less cytopathology than MuVo, and released tem