Ionic liquids bring a promise of a wide range of "green" applications that could replace conventional volatile solvents. However, before these applications become large-scale, their toxicity needs to be investigated in order to predict the impact on human health and environment. In this study we assessed the cytotoxicity of imidazolium ionic liquids (in the concentrations between 0.1 mmol L -1 and 10 mmol L -1 ) in the ovarian fi sh cell line CCO and the human tumour cell line HeLa using the MTT cell viability assay. Our results showed that the most cytotoxic ionic liquid was 1-n-butyl-3-methylimidazolium bis(trifl uoro methylsulphonyl)imide, [BMIM][Tf 2 N], followed by 1-n-butyl-3-methylimidazolium tetrafl uoroborate [BMIM] [BF 4 ], 1-n-butyl-3-methylimidazolium hexafluorophosphate [BMIM] [PF 6 ], and 1,3-dimethylimidazolium hexafl uorophosphate [MMIM] [PF 6 ]. Generally, the effects were concentrationdependent. They also depended on the type of anion and the n-alkyl chain length. The comparison between the fi sh CCO and human HeLa cell lines suggests that CCO cells provide a good biological system for initial toxicity testing of ionic liquids that could replace in vivo bioassays.