Reactive oxygen (free radical) species such as hydroxyl, superoxide or peroxyl radicals significantly affect cell function and, therefore, determine bioreactor productivities. The specific intracellular reactive oxygen species (ROS) levels as well as the ROS type, during the log and stationary phases of several Xanthomonas campestris (both untreated and HOCl-treated cells) cultivations in the bioreactor, with and without antioxidants, were measured through electron spin resonance (ESR) spectroscopy. Depending on the cultivation conditions, intracellular hydroxyl or superoxide radicals at different levels were detected. The oxygen supply during the cultivations was through need-based, pulse additions of hydrogen peroxide (H2O2), which was subsequently decomposed by culture catalase to yield oxygen. Using data from all cultivations, it was found that the growth rate constant decreased with the log-phase intracellular ROS level; the specific productivity of xanthan gum was found to be optimal at an intracellular ROS level of 0.33 mmol per g cell in the stationary phase. The theology of the xanthan gum produced in various cultivations is discussed. Further, a model, which can simulate and predict the specific total intracellular ROS level in the cultivations with untreated cells have been developed. (C) 2002 Elsevier Science Ltd. All rights reserved.