Two series of reproducible fed-batch bench scale cultures have been undertaken, one series simulating the impact of spatial variations in pH and nutrients as found at commercial scale on performance, the other, the impact of fluid dynamic stresses associated with agitation. The first was unsuccessful because, somewhat surprisingly, the use of a peristaltic pump to circulate cells and medium through different spatial environments always led to a similar reduction in culture time and resulting product titre compared to uncirculated controls. This fall was sufficient to essentially mask other effects. In the second, even at maximum specific energy dissipation rates up to similar to 160 times> with laminar extensional flow and similar to 25 times > with turbulent flow compared to typical commercial conditions, no significant effects were observed on cell growth and viability. Most importantly, in all of the cases studied, product quality was unaffected compared to controls. In addition, it is suggested that because of the possibility of cell line specific behaviour and the relationship between damage to entities and the Kolmogorov scale of turbulence, sensitivity to fluid dynamic stresses is best studied in turbulent bench scale bioreactors. (C) 2013 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.