Surface localization plays a key but ill defined role in activation of the Serum Complement System with or without related "opsonic" proteins. The adsorption of key complement components C3 and C1q and various opsonins, e. g., IgG, were therefore studied on different surfaces using in situ ellipsometry. The affinities of C3 and C1q for silica, methylated silica, and various phospholipid surfaces were shown to be largely reciprocal. While C3 adsorbed more extensively at (hydrophilic and negatively charged) silica than at (hydrophobic) methylated silica (3.1 versus 0.4 mg/m2, respectively) the opposite trend was observed for C1q (1.9 versus 2.6 mg/m2). C3 and C1q adsorbed in 10 to 15 nm thick layers on both silica and methylated silica. Each protein appeared to adsorb with consistent conformation and orientation on either surface. Adsorbed layer formation involves increased protein packing density, and molecular extension normal to the surface. Phospholipid head group properties strongly affect the adsorption of C3 and C1q at phospholipid coated surfaces. The saturation adsorption of C3 at phosphatidic acid was almost as significant as at silica, whereas the amount adsorbed at phosphatidylcholine was three times lower. C3 adsorption at phosphatidylinositol and various poly(ethylene glycol) modified surfaces was virtually absent, as was the adsorption of various opsonins. C1q adsorption was relatively low at all phospholipid and poly(ethylene glycol) coated surfaces investigated, more in the manner of IgG than C3. Preadsorption of IgG increased C1q deposition at phospholipid surfaces strongly. C3 and human serum albumin, but not C1q, showed appreciable hydrophobic affinity for a poly(ethylene glycol)-fatty acid ester of oleic acid. These results are discussed in relation to complement interaction with various surfaces and colloidal drug carriers.