Volcanic ash samples from four Mount St. Helens' volcanic eruptions were subjected to mineralogical, analytical, and hemolytic studies in order to evaluate their potential for cytotoxicity and fibrogenicity. Plagioclase minerals constituted the major component of the ash with free crystalline silica concentrations ranging from 1.5 to 7.2%. The in vitro hemolytic activity of the volcanic ash was compared to similar concentrations of cytotoxic and inert minerals. The ash was markedly hemolytic, exhibiting an activity similar to chrysotile asbestos, a known fibrogenic agent. The hemolysis of the different ash samples varied with particle size but not with crystalline silica concentration. The results of these studies taken in conjunction with the results of our animal studies indicate a fibrogenic potential of volcanic ash in heavily exposed humans.