Fibrous erionite appears to have many times the mesothelioma induction potential of asbestos. A member of a class of minerals termed zeolites, it differs markedly from asbestos in many physico-chemical characteristics. For preliminary biological studies, experiments were based on the hypothesis that the active surface sites within the internal voids are contributory to the unusual activity of this mineral. Two approaches were followed: encapsulation of isopentane into the internal spaces and exchange of cations, Cr(2+) and Ca(2+). Preliminary data suggests that toxicity to V79 cells was altered by these processes. The implication of the intense tumorigenesis of erionite may be of great basic importance in contributing data for mechanistic studies such as the illumination of a second variable in tumor induction.