The proper functioning of the mammary gland requires high-quality diet, which is based on the presence of macro- and micro-nutrients, as well as proper sanitary conditions. One of the essential and most important micro-nutrients is selenium, which is part of the enzyme glutathione peroxidase, and has antioxidant effects. The research was conducted on two groups of cows, where group I was a control group, and group II received via food 50 mg / day of selenium. According to the blood test and blood serum analysis, we noticed that in the group I selenium levels were below the physiological limits, while in group II the level of selenium was within the margin of physiological values. Also, after conducting milk somatic cell count in groups I and II, we found that the majority of cows in the group I had a somatic cell count between 310.000 and 500.000/ ml and in the group II the majority of cows had somatic cell count between 210.000 and 300.000/ ml. According to the analysis of the correlation test, we noticed that there was a negative correlation with the second group of cows, because the increasing levels of selenium in blood serum cause a decline in the number of milk somatic cells. On the basis of these results we conclude that selenium is of great importance in the preservation and proper functioning of the mammary glands of cows.