The question is addressed as to whether cells which are subject to high-energy dissipation rates in agitated bioreactors show an apoptotic response. Murine hybridoma cells in batch culture were agitated in bench-scale (1-L) bioreactors without gas sparging. At an energy dissipation rate of 1.5 W m−3 there was no apparent damage. At 320 W m−3 cell viability declined, and increasing proportions of the dead cells displayed the morphological features of apoptosis, but necrosis also remained as a significant mechanism of death. When cells were subjected to the intensive energy dissipation rate of 1870 W m−3 in a bioreactor without gas headspace, the cell number dropped by 50% within 2 h and a subpopulation of smaller-sized cells emerged. This excluded trypan blue but showed some apoptotic characteristics such as reduced and condensed DNA content and low F-actin content. The incidence of apoptotic activity was further demonstrated by the appearance of numerous apoptotic bodies. Analysis of the cell cycles of both small and normal size populations indicated that greater proportions of S and G2 cells had become apoptotic and there was evidence of preferential survival of G1 cells. It is suggested that two mechanisms of cell death are apparent in hydrodynamically stressful situations, but their relative expression depends on the energy dissipation rate. © 1995 John Wiley & Sons, Inc.