Abstract A study on cytotoxic effect of acetone extract of “Triphala” whose antimutagenicity has already been tested (Kaur S., Arora S., Kaur K., Kumar S., 2002. The in vitro antimutagenic activity of Triphala—an Indian herbal drug. Food Chemistry and Toxicology 40, 47–54) was extended to test its cytotoxic effects on cancer cell-lines using Shionogi 115 (S115) and MCF-7 breast cancer cells and PC-3 and DU-145 prostate cancer cells as models. The results revealed that acetone extract of “Triphala” showed a significant cytotoxic effect on these cancer cell-lines and the effect was similar on all cancer cell lines used in this study. The major phenolic compounds in the most potent acetone extracts were isolated and purified. Structural analysis was conducted using spectroscopic techniques including mass spectroscopy, nuclear magnetic resonance (NMR) and infrared (IR) which showed gallic acid as the major component. The suppression of the growth of cancer cells in cytotoxic assays may be due to the gallic acid—a major polyphenol observed in “Triphala”.