A methanol extract of Zanthoxylum nitidum found to inhibit topoisomerase I-mediated DNA relaxation and stabilize the covalent binary complex between the enzyme and DNA was subjected to bioassay-guided fractionation; three strongly inhibitory principles were identified including nitidine, chelerythrine, and a new alkaloid for which the name isofagaridine is suggested. © 1993, American Chemical Society. All rights reserved.