Terbenzimidazoles poison the nuclear enzyme topoisomerase I and possess significant cytotoxic activity against several human tumor cell lines. The relative pharmacological activity of 4, 5-and 5, 6-benzoterbenzimidazoles was compared to that of 5- phenylterbenzimidazole (3). 5, 6-Benzoterbenzimidazole is inactive as a topoisomerase I poison and did not exhibit significant cytotoxic activity. In contrast, 4, 5- ...