The influence of controlled- and uncontrolled-pH conditions together with the initial pH on the product and by-product distributions and oxygen transfer characteristics, whereupon the process rate limitations in relation to the intracellular reaction rates were investigated in serine alkaline protease (SAP) fermentation process by recombinant Bacillus licheniformis carrying pHV1431::subC on a defined medium with the sole carbon source glucose in the pH range of 6.80-7.25 in batch bioreactors. Although the same amount of cell was produced at all the conditions in each type of operation, with the increase of initial pH the cell formation rates increased; moreover, uncontrolled-pH operation was favourable for the cell formation. The SAP synthesis rates and concentrations were higher at uncontrolled-pH operations. Among the fermentations, pH(0) = 7.10 uncontrolled-pH operation produced maximum SAP activity that was ca. 900 U cm(-3) at t = 21 h. According to the biomass and SAP production profiles, bioprocess was divided into two periods; Period I (0 < t less than or equal to 10 h) covers the cell growth phase and Period II (10 < t less than or equal to 24 h) covers the SAP production phase. In Period I in both operations, while the oxygen uptake rate (OUR) increased with the increase in initial pH the oxygen transfer rate (OTR) decreased. In Period II, among uncontrolled-pH operations OUR was the lowest at pH = 7.10 while OTR decreased with the increase in initial pH. At the oxygen transfer condition applied, the bioprocess is biochemical reaction limited at all the conditions; nevertheless, with the decrease in initial pH, Damkohler number that is the oxygen transfer limitation decreases. Further, the perturbation effects of initial pH and the operation conditions on the intracellular reaction rates were calculated for Periods I and II by using the experimental data obtained and, the diversions in the pathways and certain metabolic reactions and potential strategies for improving SAP production are also discussed. (C) 2003 Elsevier Science Ltd. All rights reserved.