Four tetrademethyl isocolchicine analogs were prepared and evaluated as inhibitors of mammalian DNA topoisomerase in vitro. All compounds inhibited topoisomerase II-dependent DNA unknotting by a mechanism which did not involve "cleavable-complex" formation. N-Deacetylation as well as N-substitution with the (3',4',5'-trihydroxybenzoyl)-group afforded compounds which were less selective, based on their added ability to inhibit topoisomerase I-mediated DNA relaxation.