A composition for facilitating delivery of biological agents, comprising a therapeutic or diagnostic agent and a supramolecular complex, the latter comprising (i) a block copolymer, having at least one nonionic, water soluble segment and at least one polyionic segment, and (ii) at least one charged surfactant having hydrophobic groups. The charge of the surfactant is opposite to the charge of the polyionic segment of the block copolymer. The constituents of the supramolecular complex are bound by interaction between the opposite charges thereof and between surfactant hydrophobic groups. The therapeutic or diagnostic agent may be an ionic substance, in which case the ionic substance has a net charge opposite to that of the block copolymer, the net charge being no more than 10.