Different types of natural and synthetic fibres have been subjected to systematic solubility tests in vitro in a physiological solution at 37 degrees C. Both closed-system and open-system experiments were carried out. Atomic absorption spectrometry of the filtered fluids showed characteristic differences of solubility. Plastic fibres are practically insoluble. In contrast to glass fibres, the solubility of asbestos fibres is low. Sepiolite and wollastonite are of moderate solubility. The results were confirmed by scanning electron microscopy. Kinetic studies and extensive solubility tests led to a new exponential expression which describes the dissolution process in a closed system better than the square-root time laws often used. Moreover, this exponential model provides a new method of distinguishing between different materials by means of their initial rates of dissolution.