DNA topoisomerase II has been purified from bovine calf thymus nuclei using P4 phage knotted DNA as a substrate. The purified enzyme consists of two bands of molecular mass 170 and 160 kDa, as determined by sodium-dodecyl sulfate-polyacrylamide gel electrophoresis. The final product is dependent on ATP and $Mg^{2+}$, and is free of any nucleolytic, proteolytic, topoisomerase I, or protein kinase activity. A specific antibody against the amino terminal region (residues 34 to 51) of topoisomerase II has been developed. The specific antibody, designated BL1, reacts with both the 170 and 160 kDa bands of topoisomcrase II. The antibody BL1 does not affect topoisomerase II activity and the immunoprecipitates of topoisomerase II with BL1 have enzyme activity, suggesting that the 34 to 51 residues are not directly involved in the DNA binding, ATP-hydrolyzing, and DNA breaking-rejoining reactions of the enzyme. The immunoprecipitates can be used to assay topoisomerase II activity in crude extracts.