Mebendazole levels were assayed by high performance liquid chromatographic assay in plasma, host tissues, and hydatid material taken from four patients who underwent surgery for hydatid disease. The drug was absorbed and had penetrated both into the host and into the parasite material. The levels of the drug in viable hydatid cysts were much lower than those in dead cysts. The possibility of exclusion or detoxification of the drug by viable hydatid cysts is raised.