Coal fly ash of a particle diameter smaller than 10 μm was collected from the precipitator of a power plant in Hong Kong. Comparison of hemolytic activities between fly ash and free silica showed that fly ash had a lower biological effect than free silica. The hemolytic activities of the soluble and insoluble fractions of fly ash were further compared by two methods: total hemoglobin method and cyanmethemoglobin method. An analysis of results showed significant differences for fly ash and its soluble fraction between methods (P < 0.05 and P < 0.01, respectively). Fly ash, which contained a silicate level similar to its insoluble fraction, had a hemolytic activity higher than the summation of both its soluble and insoluble fractions. This indicates that the hemolytic activity was independent of the silicate content in the fly ash samples. Copyright © 1986 Published by Elsevier Inc.