The adsorption of apolipoprotein B (Ape B) at a series of surfaces was investigated with in situ ellipsometry, For silica and methylated silica, the adsorbed amount (Gamma), the adsorbed layer thickness (delta(el)), and the mean adsorbed layer refractive index (n(f)) were obtained by a procedure involving studies of the bare substrate at two different ambient refractive indices, as well as four-zone averaging. The adsorbed amount of Apo B at silica is much higher than that at methylated silica. Despite this, the adsorbed layer thickness is the same at the two surfaces, and the adsorbed layer formation proceeds similarly. In both cases, the adsorbed layer formation occurs through the adsorption of Apo B molecules in an essentially random orientation, the difference between silica and methylated silica being the number of molecules adsorbed per unit area. Furthermore, the adsorption of Apo B at phospholipid surfaces was investigated. It was found that the adsorption at phosphatidylcholine (PC) was quite limited, whereas that at phosphatidic acid (PA) was substantial. Studies with mixed PA/PC layers showed that the Apo B adsorption depends on the mixed phospholipid layer composition in an essentially linear fashion. Finally, mixed phospholipid layers of PC and ganglioside G(M1), as well as phosphatidylinositol layers, showed a dramatic preferential adsorption of Apo B over e.g., human serum albumin, immunoglobulin G, fibronectin, and fibrinogen.