With the onset of the commercial production of target proteins by hybridoma and genetically engineered cells, there is a pressing requirement for biosensors to monitor on-line and in real-time their growth in culture. In terms of irreversible thermodynamics, most of the Gibbs energy provided in substrates for this process is dissipated as heat, with only a relatively small quantity being conserved in biomass and the target proteins. Calorimetry would appear, therefore, to be a strong contender for a metabolic probe. A flow microcalorimeter was modified for use with animal cells and tested using CHO 320 cells which produce the heterologous glycoprotein dimer, Interferon-gamma. Preliminary results showed that heat flow did not follow the increase in cell-number concentration and it was realised that this technique must be combined with one to assess biomass on-line accurately to reflect metabolic activity. Dielectric spectroscopy was chosen because capacitance measurements of the culture measures the volume fraction of viable cells acid not the dead ones. A commercial version, the viable cell monitor, was validated by parallel measurements of the volume of viable cells by Row cytometry. The combined probe showed that heat flux was a function of the specific growth rate which, in turn, was related to the fluxes for glucose and glutamine utilisation and possibly to the accumulation of toxic end products, lactate and ammonia. The calorimetric-respirometric (CR) ratio was highly negative during growth in the fully aerobic conditions, indicating simultaneous anaerobic metabolism during growth. The CR ratio gave a value consistent with solely oxidative processes once there was no growth. This might indicate that ATP demand was greater than could be satisfied by oxidative processes, but this was not the case since the cells were respiring at only ca. 60% of capacity, as judged by uncoupling with FCCP. It appears likely that lactate production was due to the need to produce biosynthetic precursors which were not supplied in the medium. A better medium design would probably decrease this requirement and thereby reduce the accumulation of toxic end products.The heat flux probe was shown to be invaluable in exploring the metabolism of CHO 320 cells grown in batch culture.