Disclosed is an aqueous composition which comprises a 15-keto-prostaglandin compound as an active ingredient and contains a polyoxyethylene castor oil derivative. The solubility of the 15-keto-PG compound in water can be improved by the addition of the polyoxyethylene castor oil derivative. The aqueous composition preferably further contains a quaternary ammonium-type cationic surfactant as a preservative. The composition shows a potent antibacterial effect on the contamination of normal flora occurring in eyes such as Staphylococcus aureus
