Large differences in mesothelioma induction by various minerals were demonstrated in rats on the basis of tumor to fiber ratios. Erinoite was 500-800 times more tumorigenic and crocidolite 30-60 times more tumorigenic than chysotile. Erionite was relatively more potent when introduced intratrachealy than either asbestos. The data rank similarly to human exposure to the same mineral fibers. The authors propose that a factor here termed intrinsic potency probably related to surface activity overrides both fiber number and geometric configuration when fibers of strongly different tumorigenicity are compared. The potency difference may be related to differing tendencies of the respective surfaces to coordinate endogenous iron and release free radicals via the Fenton reaction during the reduction from the ferric to the ferrous state. It is suggested that such potency differences may explain the failure of certain man-made fibers to induce mesothelioma from inhalation exposure because of insufficient dose reaching the mesothelium. (Copyright (c) 1992 by Hemisphere Publishing Corporation.)