In batch cultures for L-ornithine production in which dissolved oxygen concentration and pH were closely controlled, time changes of redox potential were observed in connection with the profiles of cell, glucose, and ornithine concentrations. It was found that the redox potential profile had four different phases reflecting the physiological state of the culture and that it was closely related to cell concentration change. Effects of glucose and ornithine on the redox potential were identified in a separate series of experiments. On the basis of the experimental results, a correlation of redox potential to glucose, cell, and ornithine concentrations has been proposed. The proposed correlation can be used for on-line estimation of ornithine concentration from on-line data of redox potential, glucose concentration, and cell concentration.