The protein molecular assembly of calmodulin (CaM) mixed with melittin, an amphiphilic molecule, was assembled at the air-water interface by using the Langmuir-Blodgett technique. The changes in the surface pressure of the mixed protein monolayer were measured in the presence and absence of calcium ions in the subphase. A molecular coordination between both types of molecules appeared to occur in the monolayer at the air-water interface; This molecular coordination was further examined by labeling 6-p-toluidino-2-naphthalenesulfonyl chloride (CH3C6H4NHC10H6SO2Cl or TNS-Cl), a hydrophobic probe, to melittin and observing the change in fluorescence intensity in the presence and absence of calcium ion in the subphase. It was shown that molecular assemblies of calmodulin with its target molecule had a coordinated function that changed the structure of the monolayer at the air-water interface in a calcium ion responsive manner.