Earlier measurements of paraffin oil (PO)-water contact angles on protein layers [human serum albumin (HSA) or human fibrinogen (HFb)] adsorbed on polystyrene (PS) showed that HSA could be transferred from the PS/water to the PO/water interface by a passing PO-water front, while HFb could not. Interfacial association of adsorbed HFb molecules was adopted as an explanation for the irreversible localization of HFb at the PS surface. In the present work it is shown that aggregation by a heat treatment of adsorbed HSA molecules also causes an irreversible localization of the HSA at the PS surface. It is shown that advancing and receding PO-water contact angles on HSA-coated PS substrates have practically the same value. Variation of pH and NaCl concentration hardly shows any effect on these contact angles, indicating that in all cases the HSA coating behaves the same. The advancing and receding PO-water contact angles on HFb-coated substrates differ greatly. In analogy with El-Shimi and Goddard we conclude that the natural HFb coating is able to adopt and retain a configuration compatible with the immediate environment.