A suitable method for separating free drug and other residual compounds from drug-loaded, colloidal polymeric particles was developed. The objective of this separation procedure was to purify particle suspensions on a preparative scale by a rapid, simple, and exact method without changing the properties of the particles. The gel filtration method described in this paper using a middle-pressure chromatographic system is able to meet all these requirements. The separation process could be optimized by using only water as an eluent in combination with Sephadex G 50 medium. The purified particles appeared after 12-13 min in the eluate. Neither dispersity nor particle size or loading rate were influenced to a considerable extent.