Using microscopic, polystyrene-divinylbenzene beads, and an aqueous solution of sodium dodecyl sulfate and 2-mercaptoethanol, we demonstrate that fibrinogen bound to amphiphilic or hydrophobic surfaces exists as two distinct species. One species, which constitutes a small but significant percentage, similar to 10-15%, of the molecules in densely packed, adsorbed films of the protein, is not functional in the classic sense of fibrin gelation and resists detergent-facilitated elution; the other species, which constitutes the bulk of the molecules of adsorbed, saturated films of fibrinogen, is functional and elutes rapidly according to a first order process with a half-life of several minutes. A plot of the surface concentration of nondesorbable fibrinogen versus the initial surface concentration of the protein is linear with a slope near unity for low initial surface concentrations but curves and becomes constant when the initial concentrations are high. Such a profile indicates that : (1) when the area available to each fibrinogen is great, all of the bound protein takes on the form of the nondesorbable species, (2) conversion to the nondesorbable species occurs at a rate that is on the time scale of adsorption, and (3) there is a limit to the capacity of the surface for the nondesorbable species. The similarity of the molecular area of the nondesorbable species on beads, similar to 26 Angstrom(2) per amino acid residue, and that of fibrinogen in stable, insoluble monolayers at the air-water interface, similar to 23 Angstrom(2) per amino acid residue, supports the proposal that functional, desorbable fibrinogen adsorbs to the surface even as the surface is being "conditioned" by the nondesorbable species. Thus, the nondesorbable species renders hydrophilic the surface for the adsorption of the functional species, and this rendering precludes denaturation of functional molecules. We propose that amphiphilic secondary structural features of fibrinogen confer stability to the nondesorbable species, a species likely operative in the biology of fibrinogen at interfaces.