INTRODUCTION: Gold nanoparticles have immense potential as biomedical materials due to their unique geometric and biochemical properties. Studying the influence of size, shape and charge on cytotoxicity and cellular uptake is essential before future applications of these materials can be explored. We have evaluated the cytotoxicity and cellular uptake of gold nanoparticles on prostate and colon carcinoma cells due to potential utility of these particles in the treatment of cancer. METHODS: Gold spheres and rods (Nanopartz, Salt Lake City, UT) were characterized for size, shape and charge by Dynamic Light Scattering, Transmission Electron Microscopy (TEM) and UV-VIS spectrophotometry. The cytotoxicity of the nanomaterials was assessed by MTT [-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide)] and LDH (Lactate dehydrogenase) assays on human prostate cancer (PC-3) and colon carcinoma (Caco-2) cells. Internalized nanoparticles were imaged by TEM while Induced Couple Plasma-Mass Spectrometry (ICP-MS) was used to measure uptake. RESULTS: Sphere characterization revealed sizes intended by synthesis with a relatively small poly dispersity index and a highly negative zeta potential. Rod nanoparticles showed some discrepancies in size with a 10-14% standard deviation. Zeta potential measurements indicate they are highly positive. The cytotoxicity of the gold nanoparticles was found to be cell type-, concentration- and charge- dependent. Both groups of nanoparticles, rods and spheres, showed higher toxicity toward Caco-2 cells. The toxicity of the rods, in both cell lines using both methods, was much higher than the spheres, ultimately leveling off at 40-60% cell viability. The presence of cetyltrimethylammonium bromide (CTAB) on the rods was implicated as the primary reason for this toxicity. Replacement of CTAB with poly ethylene glycol significantly reduced toxicity. For both rods and spheres, no differences in toxicity were observed for differently sized nanoparticles. ICP-MS measurements showed that increasing the concentration of nanoparticles increased net uptake and decreased the percentage uptaken by the cells. Spectrophotometric measurements showed there is a spectral shift when nanoparticles were incubated with serum. CONCLUSIONS: The cytotoxicity of gold nanoparticles is concentration and cell type dependent. Rod nanoparticles containing CTAB are more toxic than spheres. The leveled off cytotoxicity at high concentration decreased percentage of nanoparticles uptaken by the cells. UV-VIS spectral shift when nanoparticles were incubated with serum might suggest non-specific serum protein adsorption onto the surface of the gold nanoparticles.