Contact angle measurements using drops of paraffin oil have been performed on polystyrene (PS) substrates, coated with human serum albumin (HSA) or human fibrinogen (HFb), immersed in buffer solution. The contact angle appeared to be time dependent. The final value for HSA-coated substrates was 50° ± 5° and for HFb-coated substrates 130° ± 10° (measured through the oil phase). From measurements of the interfacial tension at oil/buffer interfaces on passing HSA-coated substrates and from measurements using radio-labeled HSA in such experiments, it may be concluded that an adsorbed layer of HSA on PS can turnover from the PS substrate to the oil/buffer interface. The difference in behavior between PS substrates coated with HSA and PS surfaces coated with HFb is attributed to association of the HFb molecules upon adsorption at the PS/buffer interface, whereas the stronger intramolecular forces in HSA do prevent this behavior.