The invention relates to injectable long-acting insulin formulations for the treatment of types I and II diabetes in humans and animals. The essential object of the invention is to provide an injectable long-acting insulin formulation in the form of a colloidal suspension which is stable, which has a good local tolerance and toxicity compatible with the chronic treatment of diabetics, and which maintains a substantial hypoglycemic effect extending over at least 24 hours after a single administration, e.g. by the subcutaneous route. To achieve this object, the invention relates to a stable aqueous colloidal formulation of insulin-laden nanoparticles of at least one poly(Leu-block-Glu) in which the pH is between 5.8 and 7.0, the osmolarity O (in mOsmol) . . . : 270.ltoreq.O.ltoreq.800, and the viscosity v (in mPa.s) is low, namely v.ltoreq.40. The nanoparticles of poly(Leu-block-Glu) have a mean hydrodynamic diameter Dh such that: 15.ltoreq.Dh.ltoreq.40. The invention relates to an antidiabetic drug based on this long-acting insulin formulation and injectable using needles of gauge 29G, 30G or 31G.