Abstract Erionite and crocidolite fibers were tested for carcinogenicity by intrapleural and intraperitoneal injection ( una tantum) in 40 Sprague-Dawley rats per route for each fiber type. Pleural mesotheliomas were found in nine of ten animals that died within a year after intrapleural injection with erionite fibers. No pleural tumors were found among the animals treated at the same time and in the same way with crocidolite. Intraperitoneal injection of erionite did not produce mesotheliomas of the peritoneum, while a high incidence of these tumors was found among the animals treated by intraperitoneal injection of crocidolite. The studies are still in progress.