Time-dependent transitions in an adsorbed layer of fibrinogen (~300 ng/cm2) on two different poly-tether urethane ureas) were studied both by use of an attenuated total reflection infrared flow cell and by measurement of the fraction of protein removable by exposure to a 1 % sodium dodecyl sulfate (SDS) solution. The fraction of the adsorbed fibrinogen removed by exposure to SDS decreased with residence time of the adsorbed protein on the surface. The infrared spectral changes are consistent with time-dependent conformational changes in the adsorbed fibrinogen, particularly a gain in β-structures (sheets, turns, and bends). A correlation between the center-of-gravity frequency shift of the amide II band of adsorbed fibrinogen and the amount of fibrinogen retained after SDS exposure is also shown. Observed discrepancies between the two types of experiments can be explained by postulating a distribution of bound states for the adsorbed protein. © 1991, American Chemical Society. All rights reserved.