Arsenic is classified as a carcinogen for humans, but as a possible genotoxic agent. Thus, taking into account the controversial data about how arsenic compounds are able to induce genetic damage, we investigated the possible genotoxic activity of different arsenic compounds in the TK6 human lymphoblastoid cell line using the alkaline Comet assay. Eight different inorganic and organic arsenical compounds have been selected as follows: three inorganic (sodium arsenite, sodium arsenate and sodium hexafluorarsenate) and five organic (monomethylarsonic and dimethylarsinic acids, arsenobetaine, tetramethylarsonium iodide and tetraphenylarsonium chloride). According to their toxicity and genotoxicity, the highest concentration tested was 10 mM, and the duration of the treatments was 30 min or 3 h. The results indicate that some compounds belonging to both the organic and inorganic species were able to induce significant increases in the tail moment, the parameter used to determine genotoxicity. Thus, the inorganic compounds sodium arsenite and sodium arsenate (but not sodium hexafluoroarsenate) were genotoxic, while among the organoarsenic species tested only tetramethylarsonium iodide and tetraphenylarsonium chloride compounds (but not monomethylarsonic, dimethylarsinic acids and arsenobetaine) induced significant increases in the tail moment. Nevertheless, genotoxic induction was generally only observed at the highest doses tested. © UK Environmental Mutagen Society 2004; all rights reserved.