Abstract: In the present study,toxic effects of Microcystis cell extracts containing microcystin-LR on the treated-mice blood were investigated.Mice in three experimental groups received microcystin-LR by intraperitoneal injection of the cell extracts for 14 days at three sublethal doses of 2.4,4.8 and 9.6 μg microcystin-LR/kg body weight.Effects of microcystin-LR on hematological parameters and serum biological levels of the treated mice were evaluated after two weeks of administration.Mouse hematopoietic tissues,the bone marrow and the spleen were also collected for histopathological examination.Remarkable differences in liver and spleen body weight ratios between the treatment and the control were discovered.Furthermore,activities of alanine transaminase,aspartate transaminase,alkaline phosphatase,and lactate dehydrogenase in 9.6 μg/kg group markedly increased while the levels of serum total protein,albumin and albumin/globulin ratio significantly decreased compared to the control,indicating that microcystin-LR caused damages to the liver of the treated mice.In addition,severe hypoglycemia was observed in all treated mice,which was probably due to acute failure of hepatic gluconeogenesis as well as altered glucose mobilization and metabolism.In comparison to the control group,a remarkable decrease in white blood cell count in 9.6μg/kg group was observed by hematological test.Histopathological observations revealed that obvious impairment in the spleen was found in 9.6μg/kg group besides severe pathological lesions in the liver.Results presented in this paper clearly revealed that Microcystis cell extracts containing microcystin-LR had severe toxic effects on the blood of mice.