This paper considers some of the recent work at University of Birmingham on biological aspects of mixing and scale-up in pilot scale bioreactors, in particular the impact of inhomogeneities and of stress on biological performance. The work takes advantage of new experimental techniques such as flow cytometry and image analysis. The bioprocesses covered are mycelial (including GMO' s), yeast and bacterial fermentations, in batch, fed-batch and chemostat conditions. Animal cell culture is also briefly considered. By using gas blending to control dissolved oxygen concentration (dO(2)), the impact of mechanical stresses due to agitation and aeration can be decoupled from dO(2) effects. Image analysis of all the biomass shows that mycelia are generally damaged by agitation but that productivity may or may not be affected. Also, on scale-up, such damage is reduced. With yeast and bacteria, flow cytometry shows damage does not occur. On the other hand, by simulating the poorer mixing found on the plant scale that leads to locally high nutrient, low dO(2) and high pH values, a change of performance compared to the well-mixed bench-scale is found. Especially interesting is the successful simulation of the lower biomass yield but higher cell viability found on the large scale in fed-batch fermentations. Similar conclusions can be drawn for animal cell Culture, namely that provided the surfactant Pluronic F68 is added to prevent damage due the stresses associated with bursting bubbles, poor homogenisation is more a cause of poor performance on scale-up than stresses due to agitation. The paper will discuss these issues and also offer some practical recommendations concerning other important process parameters that can be measured at the pilot plant stage and the use of the information for improving large-scale performance.