Topoisomerase II was purified from Saccharomyces cerevisiae. A homogeneous preparation of the enzyme was injected into New Zealand white rabbits and the animals were boosted twice more. ELISA titers were greater than 1:10 5 at the time of the final bleed. The sera were collected and tested by probing Western blots. A single polypeptide band of 160 kDa was detected on Western blots containing crude nuclear extracts prepared from cells in mid-log phase. This is a highly specific antibody, sold as crude serum (not affinity purified) that will neutralize enzymatic activity of yeast topoisomerase II. The antibody is specific for S. cerevisiae and does not cross react on Western blots with the human or calf thymus enzymes. Although this reagent may be suitable for use in immunofluorescent applications, it has not been characterized for such applications.