The increasing production and expanding application of nanoparticles in multiple aspects of life necessitate reliable safety assessment. In this context we here report on the evaluation of the potential genotoxicity of baytubes, i.e. agglomerates of multi-walled carbon-nanotubes (MWCNT). Testing for chromosome aberrations was done in V79 cells and for gene mutations in the Salmonella microsome test. Baytubes were formulated in deionised water at 10 mg/ml and treated with ultrasound for 30 min at 25 degrees C. Particle size distribution was determined under the incubation conditions in the in vitro studies. In the chromosome aberration test V79 cells (OECD TG 473) were exposed in the absence or presence of S9 mix for 4 h to concentrations of 2.5, 5 and 10 microg/ml of baytubes (visible from concentration of 5 microg/ml and higher). Harvest was 18 h after the beginning of the treatment. In addition, cells treated with 10 microg/ml were harvested 30 h after the beginning of the treatment. An additional experiment was performed using continuous treatment at 2.5, 5 and 10 microg/ml for 18 h (no S9 mix) with subsequent harvest. Under these conditions and in the concentration range tested there were no cytotoxic and no clastogenic effects. In the Salmonella microsome (Ames) test (OECD TG 471) concentrations up to 5000 microg/plate were tested in Salmonella typhimurium (strains TA 1535, TA 100, TA 1537, TA 98 and TA 102) in the absence or presence of S9 mix. Under these conditions and in the concentration range tested there were no bacteriotoxic and no mutagenic effects.