An equation of state has been derived for a homogeneous layer of protein at the air/buffer interface. This equation includes the contribution of counterions and of the polypeptide backbone viewed as a polymer. The contribution of the polypeptide chain is derived using scaling laws. The equation has been adjusted to data obtained for spread or adsorbed layers of beta-casein. Two semi-dilute regimes are apparent: the first one between 0.3 and 1.2 mg/m2 surface concentration and the second one beyond 1.2 mg/m2. It can be concluded that the counterions do not contribute significantly to the surface pressure, that the behavior of adsorbed or spread layers is very similar, and that the ''quality'' of the solvent seems to be an important parameter of the equation. In the first regime, the thickness of the layer is constant, and in the second regime, the thickness increases. In the concentration range between 3 and 6 mg/m2, the thickness predicted by the model is close to the values measured previously by ellipsometry. A limitation of the model is the value of the correlation length at the limit between the two regimes. It seems too small compared to the size of an amino acid residue. (C) 1994 Academic Press, Inc.