Dry sedimented volcanic ash samples from each of three widely separated volcanoes of the "Circum Pacific" region have been subjected to mineralogic analysis and in vitro tests for cytotoxicity. The ash samples from the three different volcanoes varied in particle size, surface area, and concentration of silica. Total crystalline silica in the respirable fraction of ashes was 1.5% (Mount St. Helens, Moses Lake); 1.36% (Galunggung, Bandung-1); 1.95% (Gallunggung, Bandung-2); and 1.72% (El Chichon, Tuxtla). Hemolysis as an index of cytotoxicity was measured by in vitro tests on sheep blood erythrocytes and indicated wide differences in hemolytic activity among ash samples. Alveolar macrophage cytosolic (lactate dehydrogenase) and lysosomal (beta-glucuronidase and beta-N-acetyl glucosaminidase) enzymes were measured as an index of cellular integrity following dust exposure. Hemolysis and release of enzymes from alveolar macrophages were greater with volcanic ash from Galunggung (Bandung-1) and El Chichon (Tuxtla) than the other ashes. Although crystalline silica induced an effect similar to volcanic ash from Galunggung (Bandung-1) on the release of enzymes from alveolar macrophages, the hemolytic potency of silica was much greater. Light and electron microscopic observations of dust-exposed alveolar macrophages indicated that the ash particles were readily phagocytized. These results indicate that volcanic ash is moderately cytotoxic and that exposure may lead to overt reactions and the exacerbation of preexisting chronic inflammatory processes.