The effects of microcystins from cyanobacterial cells on various oxidative stress biomarkers in liver, kidney and gill tissues in freshwater tilapia fish (Oreochromis sp.) were investigated under laboratory conditions. Microcystins are a family of cyclic peptide toxins produced by species of freshwater cyanobacteria (blue-green algae). Fish were exposed to the cyanobacterial cells in two ways: mixed with a commercial fish food or crushed into a commercial fish food so that the toxins were released. Two different exposure times were studied: 14 and 21 days. The oxidative status of fish was evaluated by analyzing the level of lipid peroxidation (LPO), as well as the activities of antioxidant enzymes such as superoxide dismutase (SOD), catalase (CAT), glutathione peroxidase (GPx) and glutathione reductase (GR). The findings of the present investigation show that microcystins induce oxidative stress in a time-dependent manner and that the type of administration of the cyanobacterial cells influences the extent of these effects. Thus, the crushed cyanobacterial cells (released toxins) induced the antioxidant defences studied and increased the LPO level to a greater extent than the non-crushed cells. The liver was the most affected organ followed by kidney and gills. These results together with reports that fish can accumulate microcystins mean that cyanobacterial blooms are an important health, environmental and economic problem.