Surface pressure-average area per residue (pi-A) isotherms were obtained for the mixed monolayers of salmon calcitonin and poly[(D,L-lactic acid)-co-(glycolic acid)] (PLA/GA) spread at the air/water interface under various conditions of temperature and pH. The observed additivity of the areas/residue of the film components under all the conditions studied is attributed to the immiscibility of calcitonin and PLA/GA at the air/water interface with implications for the pharmacological properties of calcitonin-loaded PLA/GA microspheres.