Stability of dibekacin sulfate in the ointment prepared with use of macrogol ointment, white petrolatum and hydrophilic ointment as base was examined in changes in external appearance and potency. Results are summarized as follows: 1) Change in appearance: The ointment prepared from dibekacin sulfate kneaded with macrogol ointment became moist and liquified with time when stored at 25° and relatative humidity 93%. Mixture of dibekacin sulfate with white petrolatum changed color from light yellow to yellowish brown on exposure to light, and the mixture with hydrophilic ointment became light yellow on the surface. 2) Potency: The potency of dibekacin sulfate, mixed with any of the 3 ointment bases, lowered by about 10 to 15% when irradiated with light for 3 months. The lowering of potency could not be prevented by antioxidant tocopherol. Dibekacin sulfate was almost stable under other conditions of temperature and humidity.