E5880, a novel platelet activating factor receptor antagonist, was dispersed in water for use in injectable formulation and the physicochemical properties of the preparation were characterized. The critical concentration for formation of micelles was 0.12 mM. Using area per molecule data, the critical packing parameter was calculated, indicating that the structure of the micelle was spherical and that each micelle contained 49 molecules. The diameter of the micelle was 8.1 nm. Attractive interactions occurred between E5880 molecules in the micelle. The hydrocarbon region in the micelle was more rigid and less hydrated than that of other surfactants, stearyltrimethyl-ammonium chloride and cetyltrimethylammonium chloride.