Two identical laboratory-scale bioreactors were operated simultaneously, each treating an input air flow rate of 1 m(3) h(-1). The biofilters consisted of multi-stage columns, each stage packed with a compost-based filtering material, which was not previously inoculated. The toluene inlet concentration was fixed at 1.5 g m(-3) of air. Apart from the necessary carbon, the elements nitrogen, phosphorus, sulfur, potassium and other micro-elements are also essential for microbial metabolism. These were distributed throughout the filter bed material by periodic 'irrigations' with various test nutrient solutions. The performance of each biofilter was quantified by determining its toluene removal efficiency, and elimination capacity. Nutrient solution nitrogen levels were varied from 0 to 6.0 g dm(-3), which led to elimination capacities of up to 50 g m(-3) h(-1) being obtained for a toluene inlet load of 80 g m(-3) h(-1). A theoretical analysis also confirmed that the optimum nitrogen solution concentration lays in the range 4.0-6.0 g dm(-3). Validation of the irrigation mode was achieved by watering each biofilter stage individually. Vertical stage-by-stage stratification of the biofilter performance was not detected, ie each filter bed section removed the same amount of pollutant, the elimination capacity per stage being about 16 g m(-3) h(-1) per section of column.