Dipalmitoyl phosphatidylcholine (DPPC) and egg phosphatidylcholine (egg PC) are not completely miscible at all temperatures. Their phase diagram was determined by differential scanning calorimetry (DSC) of aqueous mixtures of the two. From the integrated DSX curves we obtained the enthalpy of solution of DPPC in egg PC delta hs, as a function of the mole fraction of DPPC, X, and using the empirical relationship between delta hs and X, the solubility Xsat as a function of temperature, T. The latter could be described by the semiempirical relationship: R1nXsat = a + blnT - c/T, where a = 6.57 X 10(-2) kcal.mol-1. degree -1 and c = 20.5 kcal. mol-1 (1 cal = 4.1868 J); the coefficient b was very small and could be ignored. The quantity delta hs can be given as XdeltahDPPC + deltah mix, where deltah DPPC is the gel - liquid crystalline transition enthalpy of DPPC (8.74 kcal.mol-1) and deltah mix is the enthalpy of mixing the two liquid crystalline lipids. Deltahmix depends on X in approximately a parabolic fashion, having a maximal value of 4.8 kcal.mol-1 at X = 0.6. It was shown that both the solubility and mixing enthalpy data can be described by the theory of regular solutions (RST). In RST, the activity coefficient of the solute (component 2) of a binary solution is given by RTIngamma 2 = (1 - Theta2)2deltaU, while the mixing enthalpy is given by delta hmix = Theta1 Theta2 delta U/v2, where Theta1 and Theta2 are the volume fractions of solvent and solute (egg PC and DPPC, respectively), v2 is the partial molar volume of DPPC, and deltaU is the energy change per mole on interchanging a DPPC and an egg PC molecule between their respective liquid crystalline phases. The thermodynamic data are accurately described by RST, the molar volume of DPPC being found to be about half that of egg PC solution and the interchange energy deltaU having a value of 10-11 kcal.mol-1. There was some evidence that deltaU may be an increasing function of temperature. The large value of the deltaU accounts for the pronounced temperature dependence of the solubility Xsat, which decreases from 0.35 at 35 degrees C to 0.02 at 10 degrees C. The presence of cholesterol in the mixtures decreases both the transition enthalpy of DPPC and the mixing enthalpy in a linear fashion, so that deltahs is zero at Xcholesterol greater than or equal to 0.2. The results are consistent with recent data including the formation of a PC-cholesterol complex oc stoichiometry approximately 4:1.