Composition for diagnostic or therapeutic use which comprises an assembly comprising an active agent. The assembly comprises a liposome and a plurality of micellar components associated thereto, said micellar components being associated to the outer surface of the envelope of said liposome through a substantially electrostatic interaction, When an active compound is incorporated into the micelles, a substantial amount of said active compound can be linked to a singe liposome. Furthermore, the presence of the outer micellar layer allows to increase the residence time of said liposome in the blood stream.