Food is strongly connected with the formation of free radicals in human or animal body. It is also important for the protection against them. Polyunsaturated fatty acids for instance increase the free radicals load of the organism. Antioxidants neutralize free radicals. The comet assay (Single Cell Gel Electrophoresis) was introduced for the in vivo testing of nuclear DNA damage in laboratory mice. Four groups of test animals and one control group were fed with fodder of the same basic composition but with different fats included: 1) 10% oleic acid rich sunflower oil, 2) 10% sunflower oil, 3) 10% rape oil, 4) 10% lard, 5) control group, 2% oleic acid rich sunflower oil. No antioxidants were added to the oils, lard or feeds. The energy ratio of fats in fodder was 16% in the control group and 32% in the rest test groups. Different degrees of mice leucocyte DNA damage were detected in the experiment. The highest degree of DNA damage (2.72) was found in the group fed with oleic acid reach sunflower oil. The lowest degree of DNA damage was found in the control group. Data were processed with the SAS/STAT (1990) program package. The results obtained indicate that comet assay is sensitive enough to differenciate the quality of nutritional fats from the view of free radicals formation.