The invention relates to novel pharmaceutical formulations based on fluids and stable aqueous colloidal suspensions for the sustained release of an interleukin IL- (and one or more other optional active principles), and to the applications, particularly the therapeutic applications, of said formulations. The invention aims to provide a fluid pharmaceutical formulation for the sustained release ofinterleukin(s) (and one or more other optional active principles), such that, following parenteral injection, the in vivo IL release time is increased significantly, while the plasma concentration peak thereof is lowered. Moreover, said formulation must be storage stable and, in addition, biocompatible, non-toxic biodegradable, non-immunogenic and well tolerated locally. According to the invention, the formulation is a low-viscosity aqueous colloidal suspension of submicronic particles of water-soluble, biodegradable polymer PO bearing hydrophobic groups (GH). The aforementioned particles arenoncovalently associated with at least one interleukin (and one or more other optional active principles) and form a gelled deposit on the injection site, said gelling being caused by a protein present in the physiological medium.