The effects of volcanic ash on the human complement cascade were investigated. Dusts from Mount Saint Helens, ground silica, and airborne grain dusts were added to normal human serum. Serum was tested for complement activation and monitored by electrophoresis. Factor-B was not converted at any concentration of volcanic ash. Human complement component C3 was converted to C3b by volcanic ash and silica dusts, but only moderately when compared to conversions activated by grain dusts. The activation of C3 by volcanic ash had little effect on the human complement cascade. The authors conclude that even barely detectable consumption of complement can activate significant biological activity.