A method for promoting entry of an agent (introduced agent) into a cell, the method comprising the step of complexing the introduced agent in the presence of an entry- promoting agent and then exposing to cells, wherein the entry-promoting agent comprises a linear and/or branched or cyclic polymonoguanide/polyguanidine, polybiguanide, analogue or derivative thereof according to the following Formula 1a &b. The method also provides a means for formation of nanoparticles formed between the entry promoting agent and the introduced agent. wherein: "n", refers to number of repeating units in the polymer, and n can vary from 2 to 1000, for example from 2 or 5 to 10, 20, 30, 40, 50, 60, 70, 80, 90, 100, 150, 200, 250, 300, 350, 400, 450, 500, 600, 700, 800 or 900； G
