Static and dynamic surface tensiometry, ellipsometry, and infrared reflection absorption spectroscopy (IRRAS) were used to probe adsorption of bovine (I-S) fibrinogen (FB) at the air/water interface. Tensiometry data were obtained at 25, 37, or 60°C and at concentrations from 7.5 to 750 ppm, in water, saline, or phosphate/saline buffer (PBS). The steady state tension decreased with concentration up to 75 ppm and then remained nearly constant at about 50±2 mN m⁻¹. The dynamic surface tension at pulsating area conditions, at 20 or 80 rpm (cycles per min), showed interesting and complex patterns, but tension at minimum area of 38 mN m⁻¹ or higher. Thus, if FB covers the surface, it can inhibit an essential function of effective lung surfactants, which must produce equilibrium tensions between 20 and 30 mN m⁻¹ and minimum tensions below 10 mN m⁻¹. Ellipsometry at 633 nm revealed that the adsorbed layers had refractive indices from 1.35 to 1.39, indicating a high water content, as expected from the non-compact molecular structure of the protein, which has dimensions 5×5×46 nm. The surface layer thicknesses ranged from 14 to 46 nm, and the surface densities ranged from 3 to 16 mg m⁻², indicating that if the surface layer is a monolayer, then its average orientation increases with increasing FB concentration. The thicknesses and surface densities were slightly smaller in a PBS solution than in water, with the difference increasing with increasing concentration of FB. This can be attributed to the higher stability, or weaker intermolecular interactions, of the protein molecules in the PBS solution than in water. Unlike ellipsometry and IRRAS, which probe the whole surface layer, tensiometry provides no reliable measures of adsorption densities, probably because only a small portion of the adsorbed protein molecule affects the surface tension.