Cells growing in stirred bioreactors exist in a complex fluid mechanical environment. A number of forces act on the cells in the reactor, including fluid shear stresses. If agitation is sufficiently rapid, these forces may be lethal to the cells. In this study, the effects of well defined fluid shear on cell damage were investigated in a rotational couette viscometer. The shear sensitivity of the cells was modulated by the age of the culture. For cells that experience a prolonged stationary phase, the cells were quite sensitive to shear for both young and old cultures. If the stationary phase was short, the resistance to shear was higher throughout, and declined slightly with increasing culture age. The shear sensitivity of the cells was also modulated by specific components of the cytoskeleton. Disruption of the microfilaments made the cells more sensitive to shear, while disruption of the microtubules had no effect on shear sensitivity. Shear sensitivity also depended on energy metabolism in the cells. Inhibiting respiration increased shear sensitivity, and inhibiting glycolysis caused a further increase in shear sensitivity. Addition of fetal bovine serum to the culture medium made the cells more resistant to shear in a dose dependent manner. Addition of the pluronic polyol F68 to culture media had no effect. Polyethylene glycol increased shear sensitivity. Subjecting the cells to extreme agitation over an extended period resulted in a population of cells that grew more readily at high agitation rates. This subpopulation had a higher specific growth rate and less shear sensitivity than unselected cells.