The properties of air/water adsorbed and spread monolayers of native and dephosphorylated beta-casein were monitored using surface pressure (Langmuir trough) and surface rheology (ring trough) techniques. Two stages of rearrangement are observed for native beta-casein at surface areas of about 1.0-1.3 and 0.7 m(2) mg(-1). The first accounts for distinct surface elasticity changes in the film, which are probably due to the expulsion of the most hydrophilic segments of the protein chain. The second accounts for the collapse of the monolayer. The experiments on dephosphorylated beta-casein monolayers show that dephosphorylation changes the surface elasticity behavior of the monolayer, in particular between 1 and 1.3 m(2) mg(-1). We calculated a two-dimensional Flory exponent, v, for both proteins. This exponent is constant over a (semi-) dilute range of surface tensions, maximally up to a surface area of around 1.3 m(2) mg(-1). The adsorption of native beta-casein is shown to be diffusion-limited up to a surface area of around 1 m(2) mg(-1). Experiments at high ionic strength show the importance of charge on the typical surface elasticity behavior of beta-casein. Experiments with enzymatically treated beta-casein show the importance of the presence of a hydrophilic section in the molecule on the surface elasticity behavior. It is assumed and shown that, at surface concentrations below monolayer collapse and at given solvent conditions, native beta-casein and dephosphorylated beta-casein show irreversible (air/water) adsorption behavior. Furthermore, the proteins in the monolayer are very flexible (i.e. quick relaxations).