We have studied a sample of commercial sepiolite and two samples of commercial vermiculite which are advised to replace asbestos, and we have in vitro tested their abilities to activate the complement and to lyse erythrocytes. Their behaviours has been compared with those of asbestos fibres given by Union Internationale Contre le Cancer (UICC) as reference standards. We have first measured the specific surface area and the weight of each mineral. Sepiolite has been inactive in complement activation and little able to lysis red blood cells on the base of surface area; on the base of the weight it has been unreactive in complement activations but very reactive in red blood cells lysis. We believe that in the in vitro assays to evaluate the biological effects of the substitutes of asbestos it is very important to considerate as the weight and as the specific surface are of each clay mineral.