Bacillus megaterium was used for production of the lysozyme-specific recombinant scFv D1.3 antibody fragment. Key process parameters like the temperature and the hydromechanical stress play a very important role for significant product formation during process development or scale-up. In this study, the influence of these two variables on growth and recombinant antibody fragment production in a 2-L lab-scale bioreactor system was investigated using a central composite design. Especially a significant influence of the hydromechanical stress on antibody fragment production was detected in batch cultivations. While volumetric power inputs of about 0.5 kW/m(3) (agitation rates around 500 min(-1)) are usually employed in batch cultivations, in this work maximal product concentration was found at a volumetric power input of about 0.06 kW/m(3) (agitation rate around 250 min(-1)) and at a high cultivation temperature of 41 A degrees C. The influence of the two process variables at single-cell level was estimated using flow cytometry too. The characterization was done by estimating the membrane potential giving a hint on bioprocess productivity and secretion capability: the best production was obtained through big cells with low specific membrane potential, which grew at low volumetric power inputs and high cultivation temperatures.