The adsorption of various plasma proteins to three solid surfaces has been studied as a function of plasma concentration. Albumin adsorption on glass showed no dependence on plasma concentration and increased to a plateau value on both polyethylene and siliconized glass. Immunoglobulin G (IgG) adsorption showed no dependence on plasma concentration on any surface. Fibrinogen adsorption increased with plasma concentration and then decreased, the maximum occurring at about 1% normal plasma concentration and varying somewhat with the surface. On glass the kinetics of fibrinogen adsorption was dependent on plasma concentration: at concentrations less than the adsorption maximum, the kinetics was conventional, with adsorption increasing onto a plateau; at concentrations greater than the adsorption maximum, kinetics curves also showed maxima the position of which shifted to longer times as plasma concentration decreased. These data are interpreted in terms of competitive adsorption between fibrinogen and other, as yet unidentified species in plasma. The data reported are in general agreement with the model of Vroman (12) for plasma-surface interactions which holds that initially adsorbed fibrinogen is later replaced by high molecular weight kininogen (HMWK), the rate of replacement depending on the relative activity of the surface in promoting coagulation.