Western blotting experiments with topoisomerase II antiserum were performed to measure type II topoisomerase content in human tissues. In this study, antiserum produced against the C-terminus of recombinant HeLa topoisomerase II recognized both p170 and p180 forms of the enzyme. Antisera specifically made against p180 selectively recognized that enzyme form. By using these antisera, a remarkable difference in topoisomerase content between the tissues studied was observed. Tissues with a high proportion of cycling cells contained predominantly the p170 form of the enzyme, but also smaller amounts of p180. In terminally differentiated organs or in neoplasms with a low percentage of cycling cells, topoisomerase II immunoprotein was not detected in whole tissue extracts. In term human placenta, this was because this tissue contained predominantly the p180 enzyme form, yet the concentration of this form of the enzyme was apparently in too low a concentration to be detected by the antibody in crude extracts. It was readily observed after concentration by chromatography on hydroxylapatite.