On-line estimation of cell mass and substrate concentration based on exhaust gas analysis was developed. The O-2, CO2, H2O, and N-2 contents at the inlet and outlet of fermenter, analyzed by a computer-coupled quadrupole mass spectrometer, were used to calculate the oxygen uptake rate and carbon dioxide evolution rate, and these rates were further used to evaluate cell mass and substrate concentration in a recombinant Escherichia coli fermentation. Cell mass, glucose concentration, specific growth rate, and specific consumption rate of glucose were well estimated by this method; the oxygen uptake rate gave more accurate estimates for these state variables than did the carbon dioxide evolution rate.