This study was carried out on 150 cow, their ages ranged from 6 months - over 5 years belonged to some private farms in different localities at El-Dakahlia governorate in the period from September 2004 to August 2005. The clinical examination showed 46 suspected cases which have the signs of theileria infection, laboratory parasitological examination confirmed that infection of 44 animals infected with Theileria annulata (29.33%) to total animal number where high temperature (40°-41°C), enlargment of some superficial lymph nodes, anorexia, congestion of mucus membranes, increased respiratory rates were noticed. In advanced cases of the disease signs of anaemia, general weakness were present with different degrees, in addition to presence of ticks on animal body. The study revealed that the higher rate of infection was in Autumn season (47.61%) followed by Summer season (39.47%), Spring and Winter (20%) & (5.71%) respectively. The study cleared that animals at the age of 6 months to one year have the higher rate of infection (37.50%) followed by the age of 1-3 years (32.50%), 3-5 years (27.77%) and finally over 5 years (5.88%). Concerning the changes in the blood and serum of Theileria annulate infected animals when compared with control group were as follow: Blood picture investigations proved presence of anaemia represented by high significant decrease for total erythrocytic count, haemoglobin concentration and packed cell volume and lymphocyte cells, also significant decrease for monocyte cells. On contrary presence of high significant increase in neutrophil and oesinophil cells and significant in basophil cells. Serum biochemical investigation of infected animals, showed that high significant decrease in Albumin and glucose level, while on the other side there was high significant increase in the level of both total globulin, alfa-globulin, gamma-globulin, transamenase enzymes, iron and copper, while the level of total bilirubin, blood urea, creatinine, selenium, manganese and 166 Assiut Vet. Med. J. Vol. 52 No. 109 April 2006 zinc showed significant increase. All infected animals were isolated and treated with Butalex®, Terramycin L.A and Antoplex as general haematinic. Animals and their houses were sprayed for ticks using Butox, who lead to complete recovery for diseased animals and marked improvement in haematological and biochemical parameters towards its normal levels.