Cytotoxic Activity of Tannins. Early studies have shown that several oligomeric ellagitannins exhibit in vivo antitumor activity against sarcoma 180 and MM2 in mice, which was attributed to an enhanced host immune response.21-23 In another study, oligomeric ellagitannins exhibited in vivo antitumor (against S-180 in mice) and in vitro cytotoxic (against cancer cell lines) activities; thus, direct cytotoxicity and host-mediated antitumor mechanisms were suggested.24 Recently, in vitro studies conducted with tumor cell lines have shown that several monomeric, dimeric, and oligomeric ellagitannins and their building units, the gallic and ellagic acids, exhibit potent cytotoxicity against carcinoma cell lines and lower cytotoxicity to normal cells.15,25-27 In the present study, major T. nilotica dimers (1-3), together with some related tannins (5-7 and 9) with good abundance in the plant, were tested for their possible direct cytotoxic activity on human oral squamous cell carcinoma (HSC-2, HSC-3, and HSC-4) and promyelocytic leu- kemia (HL-60) cell lines compared with their effect on human oral normal cells (HGF, HPC, and HPLF). The results, presented in