A model comprising five coupled ordinary differential equations describing the fermentative production of streptokinase in a batch reactor was reduced sequentially to one differential equation by introducing two model-based empirical functions. Four key parameters were chosen to study the sensitivity of the model, two parameters reflecting inhibition kinetics and two for decay of active cells and streptokinase. Sensitivities with respect to the inhibition parameters are dominant; perturbations in them show significant differences in the changes in sensitivities with time. These trends, and in particular the start-up profiles, provide useful indications of bioreactor performance.