Calorimetric methods have been used for investigation of the microbial activity. Investigations of a microbial culture were carried out with batch calorimeters to find a procedure for reproducible measurements and to get information about the physiological state of the microorganisms. The model organism was Paracoccus denitrificans. As a result of this work it was possible to reproduce the calorimetric experiments and provide a standard procedure for further investigations of microbial systems. The experiments showed a two-fold limitation (oxygen. glucose), which was clearly identified by calorimetric measurements under variation of the oxygen supply in combination with additional analysis (glucose, nitrate). Based on an equation for the metabolism of the cell culture the heat exchanged has been calculated and compared with the heat detected during the cultivation. (C) 2003 Elsevier B.V. All rights reserved.