Langmuir films of the lipids dipalmitoylphosphatidylcholine, docosanoic acid and octadecanol, in presence of two antibiotic ionophores, monensin sodium salt and lasalocid sodium salt have been studied. An important area increase as a consequence of antibiotic addition has been observed in the π-A isotherms of the phospholipid. The effect has been considered as a result of ionic cavity formation accompanied by rearrangements in the lipid hydrocarbon chains. A smaller area increase of monolayers from the single chain lipids has been observed only when monensin sodium salt was added. Monolayer studies of monensin sodium salt allowed molecular area estimations of the pure antibiotic in the film.