Zinc oxide (ZnO) and Titanium oxide (TiO₂) are two chemical compound with very wide industrial and commercial applications, particularly as pigments. Due to their physical properties, both compounds are also used as sunscreen ingredients for protect from UV radiation. At the nano-scale, ZnO and TiO₂ have proven to have a similar level of protection compared to normal-scale sunscreen particles. An advantage of the topical use of nano-scale ingredients in sunscreens is their transparency compared to the white residue left on skin with normal scale particles. However, the potential toxicity of these nanoparticles is not well understood. The aim of this study was to assess the cytotoxicity of ZnO and TiO₂ nanopowders (particle size 50-70 nm and less than 150nm, respectively). The cytotoxicity of these selected nanomaterials was investigated in human skin fibroblasts using the colourimetric MTS (3-(4.5-dimethylthiazol-2-yl) -5-(3-carboxymethoxyphenyl)-2-(4-sulfophenyl)-3Htetrazolium) in vitro assay. Cytotoxicity endpoints including NOAEC (no observed adverse effect concentration), IC ₅₀ (50% inhibitory concentration) and TLC (total lethal concentration) of the test materials were determined after 4 and 24h exposure. ZnO indicated higher toxicity at both 4 and 24h. By increasing the exposure time the cytotoxicity of both nanoparticles increased substantially. At 24h exposure the IC ₅₀ of ZnO was 49.56 ± 12.89 ppm, and for TiO₂ the IC ₅₀ was 2.696 ± 667 ppm. Results of this study indicated that human skin fibroblasts were sensitive to both ZnO and TiO₂ nanoparticles though the MTS assay and they correlated well with the published in vitro and in vitro toxicity data. The data generated in this study can be used to assess human topical risk exposure to selected nanomaterials.