The biological activity of Sambucus nigra bark lectin on Chinese hamster cells in vitro was investigated by comet-assay and cytotoxicity testing. Mitogenic properties at the concentrations 0.063-0.25 microg/ml (but not higher) were found, and the induction of DNA breaks at concentrations 0.5 microg/ml and higher is demonstrated. S. nigra bark lectin at mitogenic concentrations decreased the level of nickel-induced DNA damage. The character and mechanism of this lectin protective activity was probably related to the induction of DNA reparation in the cells, decreasing nickel uptake in cells, and non-specific binding of nickel ions by protein molecules.