One of the difficulties encountered in the control of bioprocesses is the lack of reliable on-line sensors for key state variables. On-line measurements of the recombinant biomass and intracellular protein are the key requirements to achieving an effective process operation. This paper investigates the suitability of using on-line estimation to predict concentrations of biomass and protein for recombinant Escherichia coli cultivated in batch and fed-batch modes. Various pairs of on-line accessible measurements of glucose, lactate, and acetate are fed back into the on-line estimator to train the tuning parameters in order to predict concentrations of biomass and intracellular protein. Such predictions are consistent with experimental data using off-line measurements. (C) 2007 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.