A clear distinction is made between damage to the population of cells and damage to individual cells on exposure to shear stress. Work on mechanical damage to animal cells in suspension is reported for six different cell lines. Precisely controlled shears of 1 Pa, 10 Pa, and 100 Pa were generated in a viscometer and distortions in morphology of the cells-for instance, the formation of transient pores, cytoplasmic extrusions, and ghost-cell membranes-are presented with photographic evidence. Low shears are shown to be just as damaging as the higher shears, although the type of damage is different. It follows that bioreactors should be operated at intermediate shear levels for optimal yield. A mechanism to account for the unexpected stability of animal cells at intermediate levels of sheer is presented. (C) 2000 John Wiley & Sons, Inc.