The operation of bioprocesses involves handling many variables with complex interactions. The visualisation of interactions is vital if suitable conclusions are to be drawn. This paper extends the previously developed 'Windows of Operation' to three dimensions, enabling the effects of three parameters to be visualised simultaneously. A tool with a graphical user interface has been developed to enable the generation of '3D-Windows of Operation' and the visualisation of the effects of the variation of any three control variables. Its use is illustrated with simulations of enzyme production. A number of control variables and the effects of the imposition of constraints on the range of possible operating values are investigated. Changes in the range possible are related to the variation in specific constraints or variables. Finally, it is illustrated how these diagrams and operation heuristics may be used to select suitable operating conditions for the bioprocess. (C) 2004 Society of Chemical Industry.