Easy enzyme tests were used to monitor the biomass activity of a trickling biological aerated filter processing a domestic influent. Biofilter wash-waters were used as biomass source. Enzyme tests (hydrolases and dehydrogenases) carried out on wash-waters showed relationships with the process active biomass (estimated by volatile suspended solids). Differences in dehydrogenase (DHA) specific activities were observed in two sampling campaigns and were Linked to process performance. The DHA activity evaluated using glucose or acetate was also related to the substrate mass applied on the biological aerated filter (kg CODt m(-3) biolite). These results indicated that, under normal operation, DHA activity is related to quantity of substrate applied to the biofilter. Similar relationships were obtained for hydrolases. However, beta -glucuronidase, Leu-aminopeptidase and protease, expressed specifically, were more significantly inversely related to process removal performance. This reaction was probably caused by the biomass reacting with an influent that is difficult to biodegrade. Generally speaking, these tests can be easily applied to the regular monitoring of the active biomass from a process using biological filters or simply as an indicator of the active biomass content in the process.