Enzymes and microbial cells are increasingly being used for a variety of biochemical transformations. During the process the enzymes/micro-organisms are subjected to hydrodynamic stresses which can lead to their de-activation. This has enormous economic impact due to their high cost. In the present work, attempts have been made to relate the extent of de-activation with the hydrodynamic stresses produced in impeller stirred reactors. The hydrodynamic stresses produced by a variety of impellers have been compared. The extent of de-activation of cellulase enzyme has been co-related with the average hydrodynamic stress within the stirred reactor.