Xenobiotics, their metabolites and transformation products are increasingly detected in the environment. The consumption of xenobiotics for human and veterinary use is on the upward path with increasing population and new diseases, and thus their release to the environment increases. With frequent monitoring of xenobiotics for human consumption, the emphasis is on increased consumption of xenobiotics for veterinary use that greatly contribute to environmental pollution. The remains of veterinary xenobiotics are detected at low concentrations in wastewaters, surface waters, groundwaters, seas and soils in a large number of countries. Given the water shortage in the world, every reuse of water is very important. It is therefore necessary to thoroughly purify waste water from all types of xenobiotics, so that their residues would not appear in the natural, and consequently, drinking water. However, data on residues of veterinary xenobiotics and their metabolites in the environment is limited, so one of the aims of this paper is to emphasize the importance of monitoring xenobiotics for veterinary, not just human consumption. Biodegradation or biological treatment is one of the classic wastewater treatment methods that is very effective in removing organic pollutants. Preliminary investigations of the possibility of biodegradation of an antihelmintic mebendazole have been carried out in this paper. Biodegradation was carried out with activated sludge from the Aglomeration Čakovec wastewater treatment plant. Biodegradability was monitored on the HPLC-DAD system. The diode array detector was used because of its simplicity and low cost, but with this method was possible monitored e only the main component mebendazole . Biodegradation of mebendazole was achieved in experiments with an activated sludge concentration of 0.29 g/L and higher. Biodegradation occurs as soon as mebendazole solution and activated sludge get in direct contact and the concentration of mebendazole is reduced below the limits of quantification. In the reactors with mebendazole an unknown component appears and it is necessary to determine whether that is the degradation product of mebendazole or an impurity from activated sludge that had reacted with mebendazole. It is also necessary to investigate degradable products, and consideration should be given to the possibility of sorption of mebendazole on activated sludge flakes.