The adsorption of Humicola lanuginosa lipase and the activity of the adsorbed lipase were studied as a function of surface wettability. The adsorption was measured by in situ ellipsometry, and the surfaces used were methylated silica surfaces. The activity of the adsorbed lipase was measured after rinsing of the cuvette, i.e., with no lipase in the bulk : solution, in situ by the hydrolysis of p-nitrophenyl acetate. The lipase adsorption and activity measurements were made at concentrations of the lipase in the range 63-1050 nM, and from the surface concentrations the specific activity was calculated. The study was carried out using fully methylated surfaces (hydrophobic; 90 degrees water contact angle) and surfaces with a higher wettability (80 degrees, 75 degrees, and 62 degrees water contact angle). All experiments were performed in 3-(N-morpholino)propanesulphonic acid (MOPS) buffer at pH 7.5. The adsorbed amount was found to be highest at lipase concentrations in the range 200-300 nM and decreased with increasing wettability of the surfaces. The fraction desorbable upon dilution of the adsorbed lipase was found to decrease with the concentration. The specific activity of the lipase was found to increase with increasing wettability of the surface, probably due to changes in the orientation/conformation of the adsorbed lipase. Additional experiments were performed where lipase was adorbed to surfaces with a water contact angle of 90 degrees, in the presence of 0.03 M CaCl2. The lipase concentrations were then 63 and 1050 nM. The amount adsorbed was significantly increased in the presence of CaCl2, but no value for the specific activity of the adsorbed lipase could be calculated due to the high activity originating from lipase adsorbed to the background surfaces (cuvette walls, Teflon tubing, and magnetic stirrer).