Accurate cell settling velocity determination is critical for pet fusion culture using a gravity settler for cell retention. We have developed a simple apparatus (a "settling column") for measuring settling velocity and have validated the procedure with 15-mu m polystyrene particles with known physical properties. The measured settling velocity of the polystyrene particles is within 4% of the value obtained using the traditional Stokes' law approach. The settling velocities of three hybridoma cell lines were measured, resulting in up to twofold variation among cell lines, and the values decreased as the cell culture aged. The settling velocities of the nonviable cells were 33-50% less than the corresponding viable cells. The significant variation of settling velocities among cell populations and growth phases confirms the necessity of routine measurement of this property during long-term perfusion culture. (C) 2010 American Institute of Chemical Engineers Biotechnol. Prog., 26: 1361-1366, 2010