Three monoclonal antibodies that react with uracil DNA glycosylase of normal human placenta were tested to determine whether one of the antibodies could be used as a negative marker for Bloom syndrome. As defined by enzyme-linked immunosorbent assay, monoclonal antibody 40.10.09, which reacts with normal human glycosylase, neither recognized nor inhibited native uracil DNA glycosylase from any of five separate Bloom syndrome cell strains. Immunoblot analyses demonstrated that the denatured glycosylase protein from all five Bloom syndrome cell strains was immunoreactive with the 40.10.09 antibody. Further, each native enzyme was immunoreactive with two other anti-human placental uracil DNA glycosylase monoclonal antibodies. In contrast, ELISA reactivity was observed with all three monoclonal antibodies in reactions of glycosylases from 5 normal human cell types and 13 abnormal human cell strains. These results experimentally verify the specificity of the aberrant reactivity of the Bloom syndrome uracil DNA glycosylase. The possibility arises that determination of the lack of immunoreactivity with antibody 40.10.09 may have value in the early diagnosis of Bloom syndrome.