Abstract: Methods and compositions for transporting drugs and macromolecules across biological membranes are disclosed. In one embodiment, the invention includes a method for enhancing transport of a selected compound across a biological membrane, wherein a biological membrane is contacted with a conjugate containing a biologically active agent that is covalently attached to a transport polymer. In one embodiment, the polymer consists of from 6 to 25 subunits, at least 50% of which contain a guanidino or amidino sidechain moiety. The polymer is effective to impart to the attached agent a rate of trans-membrane transport across a biological membrane that is greater than the rate of trans-membrane transport of the agent in non-conjugated form.