Cyclosporin-A (CsA) is a pharmaceutical product which has a polypeptide structure and immunosuppressive activity. It may be administered in the form of liposomes, and this has a series of advantages. Therefore, in this paper, the possible interaction between CsA and phospholipids is studied. The superficial monolayer technique has been used as a liposome membrane model. From the isotherms of mixed monolayers of CsA and different phospholipids it is possible to prove that these systems obey the molecular area additivity rule when spread as mixed monolayers. Applying the Defay-Crisp phase rule, it is reasonable to conclude that these systems are immiscible given that they orientate themselves differently on the surface.