We investigated the effect of cholesterol on the pressure-induced interdigitation of dipalmitoylphosphatidylcholine (DPPC) bilayer membrane by fluorescence technique using 6-propionyl-2-(dimethylamino)naphthalene (Prodan) as a probe. The interdigitation pressure (IP), minimum pressure required for the bilayer interdigitation, was elevated with increasing cholesterol composition (C-ch) up to ca. 2 mol %. Further increase of C-ch did not give any effect on the IP value. Finally, the interdigitation could not be observed at C-ch = 8 mol %. Constructed pressure-C-ch phase diagram at 50 degrees C suggests that the retardation and the abolition of the interdigitation are attributable to an interference effect and eventual formation of ordered molecular orientation induced by cholesterol.