It is generally known that the milk electrical conductivity measurement could be used for mastitis detection. REM test (Rapid electronic mastitis test) which was developed in our institute is a handheld apparatus which can measure EC of milk from each quarter of the udder. The accuracy of measurement of REM test was tested in laboratory and subsequently in practical conditions. The results have shown the accuracy was excellent (1% from measurement range). The statistical value of electrical conductivity of healthy cows was 4.8 mS/cm (sd 0.3 mS/cm) and cows with mastitis was 6.5 mS/cm (sd 0.8 mS/cm). The difference between them was significant (P < 0.01). In practical conditions absolute EC values of healthy cow changed up to 10 mS/cm between milking but the differences of EC between quarters was less than 0.6 mS/cm. Absolute value of milk EC of single quarter has substantially lesser sensitivity for mastitis detection then IQD (inter quarter difference). The differences of EC between quarters in the healthy udder were under 0.6 mS/cm. If EC of some quarter was higher than 0.6 mS/cm (opposite of the quarter with the lowest value) then the quarter was suspicious of mastitis. When EC values of all quarters in the udder were higher than 6.0 mS/cm and differences between them were lesser than 0.6 mS/cm, than the cow was suspicious of another illness than mastitis. The correlation coefficient between absolute value of EC and SCC of single quarters was r = 0.63. The sensitivity of the mastitis detection by means of milk EC measurement was calculated as 87% and the reliability 92%. It was evaluated for IQD with threshold difference 0.6 mS/cm and for cows with mastitis which have SCC > 500 000. Development of REM test was possible through projects APVV of the Slovak Research and Development Agency Bratislava (0632–10 and 15–0060), and the project CEGEZ 26220120073 supported by the Operational Programme Research and Development funded from the European Regional Development Fund.