Investigation of the oxidative stress and some heamato- biochemical changes associated with Theileriosis in cows: The prevalence of Theileria infection was investigated in 30 cows from a private farm in Fayoum Governorate with suspected symptoms of theileriosis such as rise in temperature, enlargement of superficial lymph nodes, inappetance, tachycardia, weakness. In addition, a total of 96 blood samples were collected from apparently healthy cattle before slaughtering at EL-Bassatine abattoirs. Further more, 20 positive samples for theileriosis were subjected to investigation of the oxidative stress and some heamato- biochemical changes. In addition, samples from 10 microscopically negative healthy cows were subjected to the same examinations as a control Parasitological examination revealed that the incidence of Theileria infection in the farm was 26(86.67%) and in the apparently healthy cows at EL-Bassatine abattoir was 14(14.58%) with a total incidence of 40(31.58%). Blood analysis of positive samples for theileriosis revealed a significant increase in erythrocytic malondialdehyde level (MDA) accompanied by a significant decrease of reduced glutathione (R.GSH) as well as glucose-6- phosphate dehydrogenase (G6PD). Heamatological examination showed a significant decrease in total red blood corpuscles (T.RBCs) count,  Hemoglobin (Hb%), PCV%, (packed cell volume) accompanied by a significant increase in MCV(mean corpuscular volume) in addition to significant decrease in T.L.C, (total leucocytic count) neutrophils, eosinophils and lymphocytes numbers. Biochemical analysis of positive cases revealed significant decrease of glucose, total proteins (T.P), albumin, globulin levels as well as albumin/globulin ratio (A/G), in addition to a significant increased levels of aminotranseferases, T. Bilirubin, and cholesterol, whereas a significant decrease in iron, sodium, potassium, calcium and inorganic phosphorus, were recorded compared to the control group.