Camptothecin and some of its derivatives, all inhibitors of topoisomerase I, have been found to inhibit growth and induce regression of human colon carcinoma xenografts in nude mice. Some clinical trials of these compounds have been already completed, many more are being held. The mother compound, camptothecin, which is water insoluble, has been administered orally in a Phase I clinical trial. Main toxicity encountered has been diarrhea with minimal leukocytopenia. Camptothecin is now in Phase 2 clinical trials. 9-Aminocamptothecin, a water-insoluble derivative, is now in Phase I trials. Topotecan and iridotecan, two water-soluble derivatives, have undergone Phase I trials, showing mostly intestinal toxicity, followed by bone marrow depression.