The adsorption of human serum albumin, immunoglobulin G, fibronectin, and fibrinogen at phospholipid surfaces was studied with in situ ellipsometry. For comparison, a model hydrophobic surface (methylated silica) and a model hydrophilic surface (silica) were also included in the study, as was a surface coated with an ethylene oxide/propylene oxide (EO/PO) block copolymer. The phospholipid head group composition was found to have a major effect on the serum protein adsorption. Surfaces with either no net charge or shielded charges, e.g., phosphatidylcholine, phosphatidylethanolamine, sphingomyelin, and phosphatidylinositol, or ganglioside G(M1), generally give a low adsorption of serum proteins of importance for opsonization, in analogy to the performance of adsorbed water-soluble polymers (e.g., EO/PO block copolymers), while those containing either unprotected charges, e.g., phosphatidic acid, diphosphatidylglycerol, phosphatidylserine, and silica, or hydrophobic domains (methylated silica) result in a high opsonin protein adsorption. However, the effects studied are complex, and different serum proteins generally behave quite differently at a given surface.