The adsorption of human serum albumin (HSA), IgG, 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 (PC), phosphatidylethanolamine (PE), sphingomyelin (SM) and phosphatidylinositol (PI) or ganglioside GM1, 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 (PA), diphosphatidylglycerol (DPG), phosphatidylserine (PS) 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.