Clinoptilolite is a soft, greyish-white natural rock. It contains aluminosilicates with other mixture of minerals. Clinoptilolite products are used for various purposes such as feed additives or dieatary supplements for toxin prevention in farm and poultry breeding animals (Mumpton, 1977; Harvey, 1993). Ozgur et al. (2000) used it as a bedding material for rats and also gave it as 10% and 20% clinoptilolite aqueous solution per os for 6 months and at the end of the experiments toxic effect was not observed in the rats. However, there is inadequate evidence in experimental animals for the carcinogenic potential of clinoptilolite (Tatrai et al., 1993; Adamis et al., 2000). In this study, the induction of mesothelioma was investigated by intrapleural administration of clinoptilolite.