TEPC-15 plasmacytoma cells were observed under the microscope, as they sedimented at 1 G in Dulbecco's modified Eagle's medium. A mean density of 1.0422 g/cm3 (30 degrees C) was found. The density difference of smaller cells (15 micron diameter) was significantly higher than the density difference of larger cells (25 micron diameter). The ratio in density difference was close to 3.