A tool that could determine the degradation capacity of activated sludge microorganisms for a persistent xenobiotic compound was developed. This tool was based on the color formation of degrading colonies grown on an indicator plate containing the target xenobiotic. The degradation capacity was defined as the amount of microorganisms that could degrade (degraders) among the total number of colonies (CFU). This abstract degradation capacity was quantified as the ratio of degraders to total CFU. Requirements for the tool to be reliable were met by adjusting suitable compositions of this indicator plate such that: (1) distinctive colorization for the degraders of the target xenobiotic were shown on a single plate that also gave a CFU equivalent to that grown on standard nutrient agar; (2) the number of degraders were equivalent to that shown on a plate with the xenobiotic as the sole carbon source. For the target xenobiotic 2,4-dichlorophenoxyaceticacid (2,4-D), the functional compositions that made the tool plate included the target 2,4-D: 25-300 mg/l; yeast extract: 50-300 mg/l; Eosin B: 40 mg/l and Methylene Blue: 6.5 mg/l. The tool was tested successful in quantifying the power behind 2,4-D degradation during an acclimation process. (c) 2005 Elsevier Inc. All rights reserved.