The adsorption of highly purified lipase from Humicola lanuginosa at the solid/liquid interface was monitored in situ by means of ellipsometry. Clean silica surfaces and silica surfaces methylated by exposure to dichlorodimethylsilane in trichloroethylene were used. Adsorption to hydrophobic surfaces was carried out at pH 5, 7, 9, and 10.6 and the concentrations of lipase were 2.0 x 10(-3), 2.0 x 10(-2), 0.2, 1.0, and 2.0 mg/ml. Adsorption measurements to hydrophilic surfaces were made at pH 5 and 7 at a lipase concentration of 0.2 mg/ml. In addition, the effect of ionic strength on the adsorption was investigated, and 1:1 (NaCl) as well as 2:1 (CaCl2) electrolyte was used. The adsorption of lipase was found to be highest at pH 5 and decreased with increasing pH. The amounts adsorbed increased with increasing concentrations for measurements made at pH 7 and 9 while at pH 5, high values were reached at the lowest concentration investigated and the concentration dependence was less pronounced. At all pH values, it was found that the reversibility decreased with the concentration of the lipase. No major effect from the presence of NaCl was observed. The lipase adsorption was, however, substantially increased in a CaCl2 solution.