The surface pressure-molecular area and surface potential-molecular area isotherms for mixed monolayers of valinomycin and dipalmitoylphosphatidylcholine (DPPC) at various aqueous salt subphases were measured. Two additional states of valinomycin-DPPC monolayers were observed in the presence of K+ as compared with Na+ in the aqueous subphase. The strongest interactions between valinomycin and the zwitterionic phospholipid DPPC in monolayers occur during the complexation of valinomycin with K+ from aqueous subphase.