The effects of different reaction scales [ 100 mu l reactions in 96-standard round well ( SRW) plates and 10 ml reactions in 24- square well ( SW) plates] have been investigated using, as a model, transketolase ( TK)- catalysed reaction producing L- erythrulose. Reactions were carried out under non-shaking, shaking and at 10 ml scale stirring conditions to assess the effect of diffusional limitations. Statistical analysis confirmed the significance of the observed difference in reaction rates under given conditions. Only when the laboratory scale system ( 10 ml) was well mixed did the reaction rate become comparable to that in the microwells, where there is negligible diffusional limitation. These findings have important implications for the scale- up ( or scale- down) of enzyme- catalysed reactions.