A polymerization process is described for the preparation of hydrophilic micelles containing solubilized drug molecules in a colloidal aqueous system of dissolved monomers. A hydrocarbon medium constitutes the outer phase. After secondary solubilization with the aid of selected surfactants, polymerization of micelles under different conditions takes place. The formulation of drugs including labile proteins in the ultrafine polymer components ("nanoparts") and their isolation are possible without noticeable destruction of the encapsulated molecules. Entrapped tagged material (human 125I-immunoglobulin G) shows a stable fixation in such nanoparts during long-term in vitro liberation trials. Nanoparts are visible with the aid of an electron microscope. They are mostly spherical in shape and smaller than 80 nm in diameter. They form real colloidal aqueous solutions. Nanoparts are suitable to embed antigenic material (tetanus toxoid and human immunoglobulin G) for parenteral use. These preparations show intact biological activity and high antibody production in animals.