We have purified the type II DNA topoisomerase from regenerating rat liver. The purified topoisomerase II migrated as two bands with molecular masses of 70 kDa and 55 kDa on SDS-PAGE. Immunoblotting analysis using antiserum against rat topoisomerase II gene product expressed in Escherichia coli suggested that the two bands on SDS-gel are proteolytic products of the intact 173 kDa form. However, these products retained the enzyme activities such as catenation and relaxation of supercoiled circular duplex monomer DNA and unknotting of knotted phage P4 DNA. These results suggest that DNA topoisomerase II consists of different functional domains and that the whole enzyme is not required for its activity. The activities of the purified enzyme were completely inhibited by 1 mM novobiocin, a bacterial gyrase inhibitor. However, no inhibitory effect was observed when another gyrase inhibitor, nalidixic acid was used.