Adsorption of fibrinogen onto hydrophobic and hydrophilic quartz surfaces was studied by ellipsometry and transmission electron microscopy (TEM) of negatively stained proteins. The initial adsorption at the hydrophobic surface, measured by ellipsometry, can be described by an apparent forward rate constant k1 of 2 x 10(4) M-1 s-1. This constant was time-dependent and is therefore considered as a rate coefficient. The apparent forward rate coefficient of adsorption to a hydrophilic surface was both time-dependent and concentration-dependent, indicating a history-dependent process of adsorption. Plateau levels of adsorption were concentration-dependent and lower at the hydrophilic quartz surface (1.2 pmol/cm2) than at the hydrophobic surface (1.8 pmol/cm2). These surface concentrations correspond to rather tight-packed monolayers of molecules adsorbed end-on. The initial desorption can be described by a first order rate constant (k-1 approximately 10(-4) s-1), down to 80-90% of the initial surface concentration. The dissociation rate then decreased (k-1 approximately 10(-6) s-1) resulting in an apparently stable level of adsorbed protein. Slow changes of the binding strength of adsorbed proteins was seen during 24-72 h adsorption time. Deviations from an ideal equilibrium isotherm were seen both in the time dependence and as concavities in a Scatchard plot, suggesting intermolecular cooperativity. At low bulk concentrations a heterogeneous distribution of fibrinogen molecules was found at the surface below monolayer coverage. The supramolecular structure was characterized by the formation of end-to-end dimers and trimers laying down at the surface. At higher surface concentration adsorbed molecules showed polycrystalline structure with repeated nearest neighbor distances at 16 nm. The distribution of adsorbed fibrinogen molecules indicates that surface-adsorbed fibrinogen may form a two-phase system, containing significant amounts of water. The atypical kinetics and concentration dependence of fibrinogen adsorption may thus be due to properties of a two-dimensional phase separation from a three-dimensional liquid bulk.