Lichens are recently receiving great attention because they have potential anticancer activity. Therefore, in this study, genotoxic and antigrowth properties of lichen species Usnea filipendula Stirt. were tested against human breast cancer cell lines (MCF-7 and MDAMB-231). Antigrowth effect was assayed by the MTT and ATP viability assays. Cell death modes (apoptosis/necrosis) were evaluated morphologically (fluorescence staining) and biochemically (caspase-cleaved cytokeratin 18, caspase-3 activity, and poly-(ADP-ribose) polymerase (PARP) cleavage). Genotoxic activity of U. filipendula was determined by using micronucleus, chromosomal aberration, and comet assays in human lymphocyte culture. U. filipendula inhibited growth in a dose-dependent manner and induced apoptosis by cleavage of PARP and induction of active caspase-3. It also showed genotoxic activity in doses (125 and 250 mu g/mL) higher than that required for apoptosis. These results suggest that U. filipendula may induce apoptotic cell death at lower doses, while it may be genotoxic at higher doses.