We have used both a quantitative filter binding assay and a decatenation assay to measure DNA topoisomerase II activity. The filter binding assay, which measures catenating activity, is able to detect topoisomerase II activity at 50-100-fold lower protein concentrations than the decatenation assay. Because of this remarkable sensitivity, we have been able to quantitate topoisomerase II activity in a variety of normal and neoplastic human tissues. The highest level of enzyme activity in normal tissues was found in the spleen and thymus. The highest level of enzyme activity in neoplasms was found in those that clinically behave in an aggressive manner and had a high proliferative status by flow cytometry. Surprisingly, these high topoisomerase II values in the neoplastic specimens are in the same range of values found in normal nonproliferating tissue. Since much previous data indicate that the enzyme is apparently a property of only proliferating cells, this finding might suggest that human tissues contain more than one form of the enzyme. The finding that 35-65% of the topoisomerase II activity in human tissues is resistant to teniposide suggests that more than one enzyme form exists.