The interaction of dimyristoyl phosphatidylcholine (DMPC) with sodium cholate was studied by equilibrium penetration measurements. Surface pressure-area isotherms for DMPC on substrates containing various concentrations of sodium cholate are presented. From these isotherms, the saturation adsorptions of the bile salt for various surface concentrations of DMPC were calculated. It is shown that the equilibrium penetration of DMPC monolayers by sodium cholate can be satisfactorily described by the accessible-area theory of McGregor and Barnes. A plot of the surface excess concentration of cholate against the inverse of the area per DMPC molecule is linear. This plot yields a value for the adsorption into the accessible areas of the monolayer-covered surface that corresponds to the adsorption of cholate in a monolayer-free surface. The cholate molecules lie flat on the air-water interface. The effective cross-sectional area of the DMPC in the mixed monolayer is similar to the area occupied by a DMPC molecule in a close-packed liquid expanded monolayer. The composition of the penetrated monolayer at different areas per DMPC molecule was calculated. The penetration of cholate into DMPC monolayers is correlated with different results taken from the literature dealing with the incorporation of bile salts into lecithin bilayers.