Bioluminescence was used to monitor growth of Escherichia coli in batch cultures on-line. Light emission of a strain engineered for constitutive bioluminescence was monitored with a simple set-up consisting of a photodiode, a photodetector amplifier and a recorder. Bioluminescence and colony forming units (CFU) of the cultures increased and decreased proportionally and were correlated during every growth phase at temperatures between 28 degrees C and 40 degrees C. Up to the late log (deceleration) phase, both light emission and CFU increased rapidly. Beyond the stationary phase these characteristics decreased very slowly at lower temperatures, while at higher ones they declined more rapidly. Towards the end of the cultivation, light emission of the cultures dropped to undetectable levels, even though CFU were recovered. This was particularly marked at lower temperatures where non-luminescent cultures retained very high CFU. This indicates that the actual metabolism of cells in a culture can be at a very low level or completely shut down, yet cells retain their capability to be culturable. The on-line technology described here has a number of potential uses in the laboratory and industry.