In this study, by determining in plasma the nitric oxide (NO), tumour necrosis factor-alpha (TNF-alpha) levels, adenosine deaminase (ADA), gamma glutamyl transferase (GGT) activities and percentage of T lymphocytes with alpha naphthyl acetate esterase method (ANAE), it was aimed to investigate the immunopathogenesis and biochemical mechanism of bovine leptospirosis. Blood samples of twenty healthy calves and twenty calves with leptospirosis which grown in the villages around Kars were collected into heparinised tubes from jugular vein. After taking the blood smear, blood samples were centrifuged at 3.000 rpm for 15 min. Plasma NO levels and ADA activities were determined with chemical method. TNF-alpha levels and GGT activities were determined with commercial kits. T/B lymphocyte ratios were determined at peripheral blood leukocytes with ANAE method. Plasma TNF-alpha and NO levels, ADA and GGT activities in animals with leptospirosis were found to be high P<0.001, P<0.05, P<0.05, P<0.05 respectively compared to healthy animals. ANAE positive T lymphocytes ratio were determined as 43%. As a result, TNF-alpha as a cytokine can be determined in early stage of infection and, accordingly, significantly increased levels of NO, ADA and GGT activities and, on the basis of the increased in the percentage of B lymphocytes, it is proposed to these parameters could be used for determining in explain the pathogenesis of the disease. It is tought that, it must be done more studies on biochemical mechanism of leptospirosis besides native infected cattle.