The rapid determination of waste-water quality of waste-water treatment plants in terms of pollutional strength, i.e. biochemical oxygen demand (BOD) is difficult or even impossible using the chemical determination method. The present study reports the determination of BOD within minutes using microbial BOD sensors, as compared to the 5-day determination using the conventional method. Multiple criteria establish the basis for the development of a BOD biosensor useful for rapid and reliable BOD estimation in industrial waste-waters. Of these, preparation of a suitable novel immobilized microbial membrane used in conjunction with an apt transducer is discussed. As a result, a microbial biosensor based on a formulated, synergistic, pre-tested microbial consortium has been developed for the measurement of BOD load of various industrial waste-waters. The sensor showed maximum response in terms of current difference, when a cell concentration of 2.25 x 10(10) CFU, harvested in their log phase of growth were utilized for microbial membrane construction. The sensor showed a stability of 180 days when the prepared membranes were stored at a temperature of 4 degrees C in 50 mM phosphate buffer of pH 6.8. The reusability of the immobilized membranes was up to 200 cycles without appreciable loss of their response characteristics. A linear relationship between the current change and a glucose-glutamic acid (GAA) concentration up to 60 mg l(-1) was observed (r=0.999). The lower detection limit was 1.0 mg l(-1) BOD. The sensor response was reproducible within +/-5% of the mean in a series of ten samples having 44 mg l(-1) BOD using standard a GGA solution. When used for the BOD estimation of industrial waste-waters, a relatively good agreement was found between the two methods, i.e. 5-day BOD and that measured by the developed microbial sensor.