We assessed the in vitro cytogenetic effects of extracts of the commonly used medicinal plants Equiseti herba, Ononidis radix, and Uvae ursi on irradiated human blood lymphocytes. We examined the acquired micronucleus formation in unirradiated and irradiated samples of cultured blood lymphocytes using the cytochalasin block micronucleus test (CBMN). Centromere-positive micronuclei were identified by fluorescence in situ hybridization using a DNA probe labeled with alpha-satellite digogsigenin. Equiseti herba had weak clastogenic properties, increasing the yield of micronuclei in unirradiated samples and reducing the level of radiation-induced micronuclei in a concentration-dependent manner. In the control, unirradiated samples, 36.8% of micronuclei were centromere-positive (MNC+), while in the irradiated ones the percentage of MNC+ ranged from 10.8-15.3%, indicating a clastogenic mechanism for the micronuclei formation. Ononidis radix was a strong clastogen and radiosensitizer, rapidly increasing the yield of micronuclei in unirradiated samples up to 5-fold and potentiating the yield of radiation-induced micronuclei up to 1.7-fold. In cultures treated with Ononidis radix, the percentage of MNC+ micronuclei ranged from 18.8 to 23.8%, indicating that micronuclei originated by a clastogenic mechanism. Uvae ursi did not affect the yield of micronuclei either in unirradiated or in irradiated samples. The micronucleus formation assay is a reliable screen for plant extracts and purified compounds, for the identification of compounds that might either inhibit clastogenesis or potentiate radiotherapy for malignancy.