Studies of the two-component monomolecular film system, dipalmitoyl phosphatidylcholine (DPPC) - epicoprostanol have been carried out at 21 degrees C utilizing a wide compositional range of films. The results obtained indicate that on initial mixing of the two components, epicoprostanol condensed expanded DPPC films to a greatly reduced degree and expanded condensed DPPC films to a significantly greater degree compared with cholesterol. It is thought that, even on an initial compressional procedure, partial microscopic segregation of epicoprostanol takes place. The segregation is postulated to result from an epicoprostanol-epicoprostanol overlap, leading to row packing with an accompanying reduction in the number of adjacent acyl chains. Subsequent decompression and recompression of DPPC-epicoprostanol films leads to macroscopic segregation of pure or nearly pure epicoprostanol; however, partial miscibility below 20 mol% epicoprostanol may persist. The phase diagram of the initially compressed film may be of the eutectic type, but the mixed system is unstable. The inability of epicoprostanol to substitute for cholesterol appears to be due to a combination of two effects: the tilting of the sterol at the air-water interface, to satisfy the immersion requirements of the 3 alpha-OH group, and the bent alpha-face of the sterol. The alpha-OH group of epicoprostanol is postulated to play an indirect role in the weak condensation capabilities of this sterol by inducing a tilt in the molecule and reducing epicoprostanol - acyl chain hydrophobic interactions.