Compositions are described comprising a of a block copolymer having an overall ionic charge and in which one of the blocks has pendant zwitterionic groups and a biologically active compound having a charge opposite that of the polymer. The polymer is preferably a linear diablock copolymer, preferably having a low polydispersity, such as a (tertiary amine group containing monomer) block-(zwitterionic monomer) copolymer. Suitable cationic monomers are dialkyl aminoalkyl(alk)acrylates and -acrylamides and suitable zwitterionic monomers are phosphorylcholine group containing acrylate monomers such as 2-methacyloyloxyetyl-21-trimethyl ammonium ethyl phosphate liner salt. The biologically active compound is generally polyionic and is for instance a nucleic acid, such as DNA, especially plasmid DNA.
