Interfacial tension and rheological properties conferred on the adsorbed bovine milk beta -casein and beta -casein (f114-169) were investigated in the absence and in the presence of Tween 20, a low-molecular-weight nonionic surfactant. A dynamic drop tensiometer was used with a pendant drop (if aqueous phase plunging into triolein. The equilibrium interfacial tensions determined as a function of the peptide or,beta -casein concentration were comparable. The critical interface coverage concentration of the peptide determined when the oil-water interface was fully covered was twice that of beta -casein and the dilational viscosity was higher with the peptide than with the beta -casein. The peptide could be readily rearranged at the interface and could form a densely packed layer through strong oil-peptide interactions. When the interface was fully covered by beta -casein or the peptide, Tween 20 fully displaced the protein or peptide molecules at surfactant-to-protein and surfactant-to-peptide molar ratios of 83.5 and 10.6. However, on dilation-compression of the drop, some of the protein or peptide molecules, respectively, could adsorb or desorb from the interface covered by Tween 20. This is in agreement with the mechanism of orogeny recently described. The circular dichroism investigations were consistent with local alpha -helix folding of beta -casein (f1].4-169) in some heterogeneous media.