The effect of polysorbate 40, used as a surfactant during the process of manufacture, on the properties of cytarabine microspheres with carnaubu wax was studied. It was revealed that the mean particle size increased in proportion to the concentration of polysorbate 40, and that the dissolution rate of the cytarabine from the microspheres decreased greatly more than the cytarabine powder only, but increased in proportion to the concentration of polysorbate 40 as surfactant.