Whey as a major by-product of cheese production is a subject of survey in the past decade because of its nutritional value and the possibilities to make high quality dietetic beverages. Because whey is produced in high amounts and contains organic matter in high quantity its disposal can cause serious environmental pollution. Instead the whey can be used as a raw material to produce beverages which can be classified as a healthy or functional food with added value. This is especially important for the countries with poor economic development where with a single technological process all potential of whey as a raw material can be utilized. The aim of the present study was to evaluate the whey nutritional value and the possible changes as a result of whey fermentation using kefir grains. The samples of unpasteurized cow’s whey were collected from a small farm where cheese was produced in a traditional way. After milk curdling with commercial enzymatic rennet the whey was collected and inoculated with kefir grains. The fermentation was carried out at room temperature (25 ̊C) in the span of 24 hours. The inoculation of the whey was performed with different quantities of kefir grains (5% and 10%). The examined phisico-chemical characteristics of whey were not significantly changed. The presence of lactose is an exception. The microbiological examination showed a tendency of decreasing in the number of total aerobic bacteria, E. coli and Staphylococcus aureus. As it was expected, the number of yeasts and Lactobacillus sp. increased. Additional inoculation of fermented whey with strain cultures of E. coli ATCC 8739 and coagulase positive Staphylococcus aureus ATCC9610 was performed. After 24 hours of incubation at 37 ̊C, no strains of E. coli ATCC 8739 and coagulase positive Staphylococcus aureus ATCC 9610 were recorded. These results indicated the bactericidal effects of kefir grains over the used bacterial strains but additional investigations are required.