The extent of the pulmonary reaction to intensive exposure to dust is influenced by a number of factors, in particular the nature of the fibre, morphometry, solubility, duration of dust exposure, dust concentration, and, finally, the manufacturing process of the MMMF product and its chemical composition. The biological effect of the fibres on the lung is manifest by enhanced infiltration by macrophages and an increase in the number of neutrophils. In agreement with numerous published results, our own investigations failed to reveal any signs of a fibrogenic effect of MMMF. The reason for this is presumably the lack of biostability, the tendency of the fibres to fracture crosswise, and the much lower concentration of fibres in the critical length-diameter range. In contrast, MMMF have been shown to induce an irritation of the mucosa and the upper airways. With respect to possible long-term damage produced by MMMF from our own production, we have had no such experience to date.