Isotherms of surface pressure, pi, against mean molecular area, A, were obtained for monolayers of gramicidin A (GA), ethyl palmitate (EP), and mixtures of the two spread at the air-water interface of aqueous solutions of CaCl2 at 15, 20, and 25 degreesC. Plots of A and of monolayer collapse pressure (pi(c)) against mole fraction of GA (X-GA) both suggest miscibility between GA and EP in monolayers at the working temperatures. The morphology of GA, EP, and GA/EP monolayers on aqueous NaCl and CaCl2 solutions at these temperatures was studied by Brewster angle microscopy. The two-dimensional GA/EP solution containing numerous small microdomains of condensed phase was visualized at any surface pressure.