The adsorption of fibronectin at a series of different surfaces was investigated with in situ ellipsometry. For silica and methylated silica, the adsorbed amount (G), the adsorbed layer thickness (del) and the mean adsorbed layer refractive index (nf) were obtained by a procedure involving studies of the bare substrate at two different ambient refractive indices, as well as four-zone averaging. It was found that the adsorbed amount of fibronectin was the same (1.9±0.1 mg/m2) at silica and methylated silica. However, the adsorbed layers formed at methylated silica were more extended and had a lower average protein concentration than those formed at silica. Furthermore, at both silica and methylated silica, an increasing adsorbed amount is achieved both by a denser packing of the fibronectin molecules and by a growth of the adsorbed layer normal to the surface. Furthermore, the adsorption of fibronectin at lipid surfaces was investigated. It was found that the adsorption of fibronectin to phosphatidic acid was quite significant (2.2±0.2 mg/m2), while that at phosphatidylcholine, phosphatidylinositol and phosphatidylserine was much smaller (all 0.1±0.05 mg/m2). These results are correlated to findings on the adsorption of fibrinogen at these surface, as well as on the opsonization of lipid-stabilized colloidal particles.