A duplex-PCR method, with 2 pairs of primers recognizing sequences of mitochondrial D-loop region, was developed to identify cows' milk in the milk of goats. The PCR was shown to be specific and sensitive, enabling the detection of less than 1 % of cows' milk added to the milk of goats. Simultaneous use of a primer pair for goats' and cows' mitochondrial DNA fragment prevented false negative results. The method was applied to track the presence of cow DNA in goat milk available on the Polish market. A total of 54 milk samples from 3 Polish (34) and one foreign producer (20) were examined. In 33 samples, cow DNA was detected, while 21 samples, including all of the 20 samples from foreign producers, produced the goat-specific product only.