(1) A composition which comprises erythropoietin and hyaluronic acid shows a sustained-release of the medicine in a living body, and(2) A water-soluble composition which comprises (a) a pharmacologically active polypeptide secreted by an animal body or its derivative or a chemically synthesized pharmacologically active substance, (b) a water-soluble species of hyaluronic acid or its nontoxic salt and (c) a water-soluble protein injectable into body fluids without showing any substantial pharmacological activity brings about a prolonged action in vivo of a pharmacologically active substance. In addition, the composition can be administered using a small-gauge needle and thereby contributes ot relieving pain in patients.