Immune antiserum to uracil-DNA glycosylase was obtained by immunizing rabbits with an enzyme isolated from the rat liver. Antiserum was found to suppress the activity of uracil-DNA glycosylase not only in the extracts of rat liver, but also in the extracts of brain, cardiac muscle, kidney, spleen, thymus of rats, and in those of human placenta too. This enables us to make a conclusion about the similarity in antigenic properties of the enzyme in cells of various types of differentiation. Indirect immunofluorescent test shows a slight staining of the periphery of the nucleus in normal liver hepatocytes and the intensive staining of the inner part of the nucleus in hepatocytes of regenerating liver. Therefore it is concluded that the enzymatic activity increases as cells proliferate. This may be the result of the appearance of uracil in DNA during replication.