"Milk is the most perfect natural food!", said the Greek doctor and philosopher Hippocrates around 400 BC. Dairy products have long been known for their beneficial effects on human health. In addition, they are rich in biologically active ingredients, often mentioning their effects of neutralizing free radicals, which, provoked by the oxidative stress, often cause the development of acute and chronic diseases, malignant tumors, cardiovascular as well as inflammatory diseases. The aim of this research was to investigate and compare the antioxidant activity of fermented dairy product – kefir obtained using kefir grains, prepared from raw and durable (UHT) cow and goat milk for 24 h and 48 h. Using the reduction of 2,2-diphenyl-1-picrylhydrazyl (DPPH) radicals, the antioxidant activity of samples was measured by electron spin resonance (ESR), a spectroscopic technique that can precisely and accurately determine the concentration of radicals in a sample. The DPPH solution was irradiated in methanol at the concentration of 0.15 mmol/L, and the loss of the ESR signal intensity, relative to the initial one, was measured during fifteen minutes. Signal intensity, calculated by the double integration of the ESR spectra, is proportional to the number of radicals in a sample and therefore to its antioxidant activity. The results of this study show that milk and fermented dairy products, due to their origin, time and unlike chemical-mineral composition, have different antioxidant properties. Likewise, the fermentation and its duration, as well as the treatment of the milk from which kefirs are prepared with the help of kefir grains, have an effect on the inhibitory activity of antioxidants against the DPPH radical.