Toxin-producing cyanobacteria pose a worldwide health threat to humans and animals due to their increasing presence in both drinking and recreational waters. Little work has, however, been done on a preventative therapy for anyone at risk of exposure to cyanobacterial toxins. The potential benefits of dietary supplementation of selenium, an antioxidant, to protect against the mouse liver injury induced by the toxin, microcystin-LR, has been investigated. BALB/c mice were pretreated for two weeks with sodium selenite (1.5 microg/mouse/day) before an intraperitoneal injection of microcystin-LR. Selenium-supplementation was found to provide some protection to the action of the toxin. In addition selenium pretreatment reduced the liver damage caused by lethal and sub-lethal toxin doses as reflected in liver pathology, decreased serum ALT and lipid peroxidation levels as well as prevention of glycogen loss compared to non-selenium supplemented toxin treated mice. The increased level of liver glutathione peroxidase activity following selenium-supplementation may indicate the possible route of selenium protection in the mice.