The adsorption mechanism of the alkaline protease Savinase from Bacillus lentus has been investigated. Adsorption isotherms for solid-liquid interfaces were determined and the interactions of importance for the adsorption of this protease were examined by varying (\mathrm{pH}), ionic strength, temperature, and nature of the surface. The adsorption of Savinase appeared to be determined by electrostatic interactions between the surface and the enzyme, dehydration of hydrophobic parts of the enzyme and substrate surfaces, and lateral repulsion between the adsorbed Savinase molecules. The contribution of the dehydration of hydrophobic surfaces to the driving force is less than that for the adsorption of proteins, such as RNase, reported in the literature. It was concluded that the enzyme adsorbs without unfolding on the interface. As the adsorption strongly depended on the electrostatie interactions between the surface and the enzyme, the contributions of electrostatic lateral repulsion and dehydration of hydrophobic parts of the surface to the Gibbs energy are on the same order of magnitude. The range of the lateral repulsion extends beyond the Debye length. The relative importance of the electrostatic interaction resulted in a decrease of adsorption on a negatively charged surface with increasing (\mathrm{pH}). Under attractive electrostatic conditions the adsorbed amount decreased with increasing ionic strength. The adsorption proved to be dynamic (protein molecules readily replaced adsorbed ones), although this exchange was not reversible in the case of dilution. On a hydrophobic surface the adsorption was reversible toward changes in pH and ionic strength. 1995 Academic Press, Inc.