This study was an investigation of the cytotoxic activity of iridoid glucosides, including aucubin, catalpol, 6-O-acetylcatalpol, veronicoside, catalposide, verproside, amphicoside, veratroylcatalposide, verminoside, aquaticosides B and C isolated from different Veronica species. The cytotoxic activity was determined against Hep-2 (human epidermoid carcinoma), RD (human rhabdomyosarcoma), L-20B (transgenic murine L-cells) cancer cell lines and Vero (African green monkey kidney cells) non-cancerous cell line using the MTT method. While verminoside, amphicoside and veronicoside were found to exhibit cytotoxic activity in the concentration range of 70–355 µ m, acetylcatalpol, aquaticosides B and C, catalposide, veratroylcatalposide and verproside showed cytostatic activity. Apoptotic cell death was observed as the effect of verminoside in the histological analysis of the tested cell lines. In conclusion, iridoid glucosides are considered to show a biphasic effect on cancer cells that is both cytostatic and cytotoxic, depending on the chemical structure and the type of cancer cell. Copyright © 2011 John Wiley & Sons, Ltd.