The measurement of cell proliferation and cell viability has become a key technology in the life sciences. The need for sensitive, reliable, fast and easy methods has led to the development of several standard assays. These include the determination of DNA synthesis by measuring the amount of radioactive labeled nucleosides like [3H]-thymidine incorporated in nucleic acid. Alternatively several tetrazolium salts, including MTT, XTT and MTS are usually used to assay cell proliferation and viability on microplate assays. Actually there is an immediate need for nanotoxicology the availability of rapid and inexpensive tools in order to carry out some preliminary test for nanomaterials and the aim of our work was to investigate on the feasibility of WST-1 assay for this purpose. In our aim we investigated the toxicity in term of cell proliferation of toner nanoparticles (5-50 nm) on human bladder carcinoma cell line RT112 by WST-1 assay and also to evaluate the feasibility to apply this method as tool for in vitro nanotoxicology studies. In order to relate the toxicity test results to the toner powder properties, chemical and physical-chemical analyses have been performed in parallel on the same batch sample. © 2009 IEEE NANO Organizers.