In order to investigate the influence of molecular size and pyrrole (Py)/imidazole (Im) content on the cell permeability of Py-Im-polyamide-fluorescein conjugates we systematically designed the Py-polyamides and Im-polyamides. Flow cytometric analysis revealed that Py-polyamides, even those with large molecular size, P-15 and P-18, showed good cellular uptake, but Im-polyamides showed very poor uptake. Fluorescence microscopy revealed that conjugate P-6 exhibited nuclear localization, while P-18 showed less nuclear stain but intracellular localization, suggesting that increased molecular size is one of the determinants in reducing nuclear access. Furthermore, results for hairpin polyamide conjugates H-1, H-2, and H-3 containing different Py/Im content indicated that cellular uptake increases as the Im residue is reduced. It appears that Py-Im-polyamide has general properties regardless of whether they have a linear or a hairpin structure. (C) 2009 Elsevier Ltd. All rights reserved.