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: &ldquo；n&rdquo；, 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； G1 and G2 independently represent a cationic group comprising biguanide or guanidine, wherein L1, and L2 are directly joined to a Nitrogen atom of the guanide； L1 and L2 are linking groups between the G1 and G2 cationic groups in the polymer and independently represent an aliphatic group containing C1-C40 carbon atoms, for example an alkyl group such as methylene, ethylene, propylene, C4, C5, C6, C7, C8, C9 or C10； C1-C10, -C20, -C30, -C40, -C50 -C60 -C70, -C80, -C90, -C-100, -C110, -C120, -C130 or -C140, alkyl； or a C1-C140 (for example C1, C2, C3, C4, C5, C6, C7, C8, C9 or C10； C1-C10, -C20, -C30, -C40, -C50 -C60, -C70, -C80, -C90, -C100, -C110, -C120, -C130o or -C140), cycloaliphatic, heterocyclic, aromatic, aryl, alkylaryl, arylalkyl, or oxyalkylene radical； or a polyalkylene radical optionally interrupted by one or more, preferably one, oxygen, nitrogen or sulphur atoms, functional groups or saturated or unsaturated cyclic moiety； N and G3 are optional end groups； X can be either present or absent； L3, L4 and X are linking groups between the G4 and G5 cationic groups in the polymer and independently represent an aliphatic group containing C1-C140 carbon atoms, for example an alkyl group such as m
