A theory based on frustrated phagocytosis is advanced to explain the carcinogenesis of fibers and films. Exocytosis combined with flexion of a foreign body permits the escape of superoxide radicals which, in a manner similar to that postulated for ionizing radiation, induces somatic mutations. The theory is subject to experimental verification, and offers hope for the development of preventive therapy which can be used to prevent some of the fibrosis and cancer resulting from asbestos inhalation.