The heat flux of animal cells growing in vitro is a direct measurement of their specific metabolic activity because the majority of the substrate Gibbs energy is dissipated as heat with only a small quantity being dissipated as entropy conserved in the form of biomass. Heat flux can be used as a variable to monitor the growth of engineered cells producing heterologous proteins in large-scale, industrial culture. Indeed, since heat flux is a function of metabolism, it can be used as the control variable in fed-batch culture. Until now, monitoring a culture by circulating a cell suspension to a standard thermometric TAM how calorimeter has been compromised by direction of flow, slow maximum flow rate, narrow bore tubing and small volume of the measuring vessel. A new twin flow module for the TAM calorimeter is described which is optimised for use with cell suspensions as well as for cells growing on microcarrier beads. It consists of a continuous length of stainless steel tubing of 1.5 mm ID which forms the transmission lines, the heat exchangers and the coiled measuring vessel nominally of 1 cm(3) in volume. The heat exchangers are of sufficient volume to permit fast pumping rates (<200 cm(3) h(-1)) while the increased size of the vessel allows for a lower detection limit. The module is designed for downward flow through this vessel. It also has a calibration heater (nominal 50 Ohm resister) between the fine heat exchanger and the measuring vessel. The detector is downstream of this vessel and consists of a thermopile operating on the heat conduction principle. The flow module was chemically calibrated by the exothermic hydrolysis of triacetin in imidazole-acetate buffer At 100 cm(3) h(-1), the effective thermal volume was found to 1.05 cm(3). For validation, the experiments were on the growth of CHO320 cells producing recombinant interferon-gamma in batch culture and under the steady state conditions of continuous culture. The on-line heat flow rate was smoothed by the moving-average technique and showed an increase proportional to cell growth during the batch phase and only small changes after setting up the continuous culture. Repeated experiments gave consistent results without signs of erratic recordings and thus indicated that the specially designed flow module can indeed realise the potential of heat flux to monitor metabolic activity in batch and perfusion cultures together with being a control variable in fed-batch cultures.