The importance of the physical characteristics, rather than the chemical composition of fibres, in relation to their alveolar deposition and biological effects is stressed. For alveolar deposition in the aerodynamic properties--depending on the fibres diameter--are decisive, whereas in the interaction between macrophages and fibres, fibre length is the decisive factor for cytotoxicity. Probably fibre length could, inter alia, be responsible for a pathogenic effect around the lung tissue. The results from measurements of the cytotoxicity of the UICC standard asbestos dusts and pure cement dust are presented and discussed.