Since 1986, there have been reports in the literature that glycyrrhizin (GL), 18 alpha-glycyrrhetinic acid (18 alpha-GA) and 18 beta-glycyrrhetinic acid (18 beta-GA) can inhibit the growth of some murine tumours. Recently, testing the activity of GL and 18 alpha-GA on human tumour cells, we observed that both substances are good anti-proliferative agents, especially on those cells whose replication rate is slow. In the present study, the MTT test was performed on MCF-7, Hep-2 and VERO cells treated with increasing concentrations of GL and of its derivatives. Results showed that the substances do not have a toxic effect on VERO cells and that their anti-proliferative effect is evident only on the cell line (MCF-7) that evolves slowly. Finally, the TUNEL test revealed the presence of apoptosis in MCF-7 cells treated with dosages of GL and 18 alpha-GA, thereby confirming that these natural phytoestrogens could be considered as interesting new agents for cancer therapy.