The fibrogenic properties of Mount Saint Helen's volcanic ash were examined in Fischer-344-rats. A dry, sedimented sample of volcanic ash from the first eruption was collected at Spokane, Washington within a few hours of sedimentation. Mineralogic analysis and particle sizing was conducted using electron microscopy, X-ray analysis, X-ray powder diffraction, and image analysis. Animals were given intratracheal applications of 1 or 10 milligrams volcanic ash and were sacrificed and examined at days 1, 7, 28, 56, or 180. Lung tissue was microscopically examined. Scanning electron microscopy showed that the majority of ash particles had smooth surfaces and sharp, angular borders. No fibrous minerals were identified in the ash. Between 81 and 99 percent of ash samples was in the respirable range, with an average area of 1.7 micrometers. The majority of the particles were plagioclase minerals; total crystalline silica (7631869) was 7.2 percent. All exposed animals survived the experimental period. Several significant morphological changes were seen, including acute inflammatory cell response on day 1, which became predominantly mononuclear by day 7. Well formed granulomas were seen at day 28; these were scattered throughout the lung parenchyma. Mild thickening of the walls of respiratory bronchioles and alveoli was observed; ash was seen in association with the lesions. At 180 days, there was widespread granulomata in both lungs. Distribution of lesions was uneven at all time intervals. Animals given 1mg ash showed similar but less severe or extensive lesions. The authors conclude that the formation of granulomata with areas of interstitial fibrosis in exposed rats shows that volcanic ash is fibrogenic in-vivo.