AbstractTwo varieties of native and chemically treated slate dust were testedin vitrofor their hemolytic effects and the extent of silicic acid dissolution in various physiological fluids. The extent of hemolysis was found to be proportional to the degree of dissolution of dust constituents. Membrane lysis by the dust appeared to be prevented by coating it with polyvinyl pyrrolidone, serum proteins and pulmonary lavage lipids. The significance of the findings is discussed.