The presence of acidic sites on mineral dusts found to be inhibitors of the enzyme human leucocyte elastase (HLE) has been investigated by means of calorimetric and spectroscopic (IR and EPR) techniques. Two acidic phyllosilicates (montmorillonite and kaolinite), which were the most potent inhibitors, and a basic one, muscovite (poor inhibitor) have been examined from the standpoint of their ability to strongly bind ammonia from the gas phase and to adsorb arginine from aqueous solutions. A correlation between inhibiting potency, strong surface acid sites (heat of adsorption of ammonia above 100 kJ mol(-1)), and arginine adsorption is envisaged. Chrysotile, a magnesium phyllosilicate, is also a rather poor inhibitor of the enzyme. When the magnesium hydroxide layer is partly leached out, its affinity toward basic molecules and its inactivating potency increase. A model is proposed whereby the enzyme is inactivated when strongly adsorbed on surface acidic sites-dissociated in the aqueous medium-arising from polarized water on small cations (mainly aluminum) and/or particularly acidic surface hydroxyls. This is consistent with the reversibility of the inactivation upon variation of pH and increase in ionic strength.