Defensin-A is an inducible antibacterial protein isolated from the larvae of Phormia terranovae which interacts with membrane cells by forming ion-conducting pores. Defensin A adsorbs at the air-water interface from an aqueous solution and is able to spread as a monolayer. The shape of the compression isotherm curves of defensin monolayers is very sensitive to the pH of the subphase. Defensin A penetrates into lecithin monolayers. In mixed monolayers defensin A and lecithin are at least partially miscible. Plots of the mean molecular area as a function of composition show positive deviations from the additivity rule. The conformational state of defensin A at the air-water interface undergoes a change upon interaction with phospholipids.