Gold nanorods were synthesized using a seed-mediated wet chemical approach with a quaternary ammonium surfactant, cetyltrimethylammonium bromide (CTAB), that forms a bilayer on the surface of the nanorods. The CTAB molecules in the bilayer were exchanged with a similar polymerizable analog, 11-(acryloyloxy) undecyltrimethyl ammonium bromide (p-CTAB). Mass spectrometric analysis of the degree of exchange of CTAB for p-CTAB, after gold digestion, gave 77 +/- 3 and 23 +/- 1% for p-CTAB and CTAB, respectively. On-rod polymerization with a cationic free-radical initiator was confirmed by FTIR analysis and did not induce aggregation as judged by ultraviolet-visible spectroscopy, transmission electron microscopy, and dynamic light scattering measurements after polymerization. In contrast to the nanorods before polymerization, the nanorods with a polymerized bilayer showed improved stability against dialysis as well as enhanced biocompatibility as measured using a viability assay on cultured human cells. Our results indicate that (1) CTAB molecules on the surface of the gold nanorods are exchangeable with similar surfactants that have a positively charged headgroup and (2) surfactant polymerization on the surface of the gold nanorods enhances both the stability and biocompatibility of these nanomaterials, probably by decreasing the degree of surfactant desorption from the surface.