BACKGROUND: An unstructured mathematical model was developed to understand information on the relationship between Bacillus circulans growth and metabolism-related protease production (using logistic and Luedeking-Piret equations respectively) in a batch reactor with respect to glucose consumption and fermentation time. The objective was to develop an indispensable tool for the optimisation, control, design and analysis of alkaline protease production. RESULTS: Biomass growth and enzyme production titres changed with a change in substrate concentration. Modelling analysis of biomass and enzyme production titres at different substrate concentrations revealed significant accuracy in terms of statistical consistency and robustness with respect to fermentation kinetic profiles. CONCLUSION: With the B. circulans strain used, an economic protease yield (2837 x 10(3) U g(-1)) with respect to biomass and glucose ratio was achieved at low substrate concentration (110 g L-1). The developed model could be effectively utilised for designing, controlling and up-scaling the protease production process in high-density fermentation in selected bioreactors with statistical consistency. (C) 2008 Society of Chemical Industry