For the proper functioning of the mammary gland before the sanitation requires is a quality diet based on the presence of macro and micro nutrients. One of the essential and important micro-nutrient is selenium, which became part of the enzyme glutathione peroxidase, and has antioxidant effects. The research was conducted on thirty dairy cows Holstein breed, who received 0,3 mg/kg selenium supplementation in food daily. Samples were colected two times: at first and at sixth lactating months. The mean estimate of selenium blood concentration at first lactating month was 0.536μmol/l and at sixth lactating month was 0.601μmol/l. Average somatic cell count at first lactating month was 450.000/ml of milk and at sixth lactating month was 355.000/ml of milk. According to the analysis of the correlation test, negative correlation within blood selenium concentration and milk somatic cell count has been found . The increased levels of selenium in blood caused a decline in the number of milk somatic cell count. On the basis of these results it could be concluded that selenium is of great importance in the preservation and proper functioning of the mammary glands of cows.