MOPC-315 murine myeloma cells were successfully adapted to growth in culture and the established cell line propagated in suspension and retained the capacity to induce plasma cell tumors in mice. The cultured myeloma cells constantly release RNA viral particles which by nucleic acids hybridization are indistinguishable from the C-type particles secreted into the ascitic fluid of mice bearing MOPC-315 myeloma tumors. Availability of an in vitro source of MOPC-315 murine myeloma viral particles facilitated additional biochemical characterization of these viruses. The myeloma particles were found to be infectious and capable of inducing syncytia in XC cells. The particles released in culture, unlike those secreted in vivo, showed high instability during the purification steps, but this was partially overcome by the addition of dithiothreitol.