Abstract: Polymer-cargo-complexes including cross-linked copolymers and cargo molecules bound to the copolymers by electrostatic interactions. The non-medical use thereof for transfection and to a kit including such polymer-cargo-complexes. The polymer-cargo-complexes for use in therapy, in particular for use in gene therapy or peptide/protein drug delivery. A method of preparing the polymer-cargo-complexes. A kit for preparing a polymer-cargo-complex of the invention.