Isothermal reaction calorimetry was employed on the bench scale to measure the enthalpy efficiency and O2 efficiency of growth of several microbial strains cultured on a range of substrates with varying degrees of redn. The obsd. values of these energetic efficiency factors agreed well with simple theor. predictions based solely on the degree of redn. of the substrates as independent variable. Calorimetry is thus shown to be a useful tool for investigations of microbial growth energetics. Measuring heat dissipation in large scale bioreactors provides online information on the energetics of growth and should therefore be useful for process control purposes. [on SciFinder (R)]