The use of an artificial electron acceptor in microbial respiration was investigated with a view to developing a rapid assay for biochemical oxygen demand (BOD). The use of ferricyanide resulted in a significant increase in the rate of the biochemical reaction and allowed for biodegradative conversion efficiencies similar to the 5-day BOD assay to be achieved in 1 h. The extent and rate of the ferricyanide mediated microbial reaction was determined by monitoring the concentration of the microbially produced ferrocyanide during or after incubation of microorganisms in the presence of ferricyanide and organic substrate. Spectrophotometry, potentiometry and amperometry using microelectrodes were evaluated as detection methods, with the latter providing the most convenient, stable and reproducible results. Experimental parameters investigated included incubation time, incubation temperature, microbial concentration, ferricyanide concentration and substrate concentration. In all cases, the results obtained were analogous to that expected in conventional aerobic microbial oxidation of organic material, with the major difference being the considerable increase in rate. The microorganisms used in this study were Escherichia coli and Pseudomonas putida. Results showed that while E. coli could successfully catabolise a standard BOD solution containing glucose and glutamic acid, its use for other substrates was limited. Preliminary investigations into the use of P. putida, however, showed significantly improved performance and demonstrated the promise of this approach for rapid BOD determinations