A method of producing a sustained-release preparation which includes permitting a water-soluble polypeptide to permeate into a biodegradable matrix in the aqueous solution. The production method of the present invention makes possible the permeation of a water-soluble polypeptide into a biodegradable matrix without bringing the water-soluble polypeptide into contact with an organic solvent. Hence the water-soluble polypeptide is prepared without affecting the water-soluble polypeptide bioactivity and is thus effective for use as a pharmaceutical.
