Bacterial L-asparaginases are amidohydrolases that catalyse the conversion of L-asparagine to L-aspartate and ammonia and are used as anti-cancer drugs. The current members of this class of drugs have several toxic side effects mainly due to their associated glutaminase activity. In the present study, we report the molecular cloning, biochemical characterisation and in vitro cytotoxicity of a novel L-asparaginase from the pathogenic strain Helicobacter pylori CCUG 17874. The recombinant enzyme showed a strong preference for L-asparagine over L-glutamine and, in contrast to most L-asparaginases, it exhibited a sigmoidal behaviour towards L-glutamine. The enzyme preserved full activity after 2 h incubation at 45 degrees C. In vitro cytotoxicity assays revealed that different cell lines displayed a variable sensitivity towards the enzyme, AGS and MKN28 gastric epithelial cells being the most affected. These findings may be relevant both for the interpretation of the mechanisms underlying H. pylori associated diseases and for biomedical applications.