Lack of participation of murine myeloma cells in the fusion of cocultured XC cells.

Electron microscopy of thin sections of syncytia, formed by fusion of homologous XC cells, clarified that these heterologous cells were simply invaginated by syncytia and that they were clearly separated by the limiting cytoplasmic membranes. The C particles found at the boundary showed no sign of connection with the surrounding syncytial cell membrane, while some were seen budding from the surface of the producer cells. These results strongly suggest that the XC cells, even when cocultured with the C-type virus-producing myeloma cells, fuse homologously among themselves, and the virus-producing heterologous cells do not participate in the event.