The invention relates to an amphiphilic-BMP polymer complex which is physically and chemically stable and soluble in water, characterised in that: the amphiphilic polymers comprise a hydrophilic polysaccharide skeleton functionalised with hydrophobic substituents and hydrophilic groups the BMP is selected from the group of therapeutically active BMPs (Bone morphogenetic Proteins), the polymer/BMP mass ratio is lower than or equal to 700. The invention also relates to the process for preparing the amphiphilic-BMP polymer complex in an aqueous medium and in the absence of organic solvents likely to denature the protein. The invention also relates to therapeutic compositions of an amphiphilic-BMP polymer complex according to the invention.