The hemolytic capacity of volcanic ash was assessed in sheep erythrocytes. Ash samples from four eruptions of Mount Saint Helens in Washington state were collected at various sites. Some samples from the first eruption were fractionated. Crystalline silica (14808607) content was determined. Particles were sized and identified then compared with nine mineral dusts on the basis of known biological properties. Volcanic dust or other minerals were suspended with 2 percent sheep erythrocytes and incubated for 50 minutes at 37 degrees-C. Optical densities of supernatant solutions were read at 541 nanometers. The majority of the ash particles had flat surfaces with angular edges. Free crystalline silica content varied greatly among the samples, depending on site and eruption. Plagioclase minerals, including sodium (7440235), silicon (7440213), aluminum (7429905), potassium (7440097), and calcium (7440702) accounted for most of the material contained in the ash. The respirable size range of less than 10 micrometers accounted for 99 percent of particles. A concentration of 12.5 milligrams per milliliter (mg/ml) volcanic ash induced a 64 percent hemolysis, compared with crystalline silica, with a 94.3 percent hemolysis, and chrysotile and talc with 67.9 and 57.9 percent hemolysis, respectively. All ash samples showed increased hemolytic activity with increased concentration. At a concentration of 12.5mg/ml, ash samples from the second and fourth eruptions were about 50 percent lower than the other samples. The authors conclude that the volcanic ash samples from different locations and eruptions exhibited hemolytic activity similar to that of chrysotile asbestos (12001295). Degree of hemolysis was more dependent on particle size than on free silica content.