The aims of this paper were to investigate cell growth activity of fullerenol C60(OH)24, its modulating effect on antitumor drug-induced cytotoxicity, and genotoxic influence of fullerenol at nanomolar concentrations. Human breast cancer cell lines, MCF-7 and MDA-MB-231, were treated with fullerenol at concentrations from 0.9 to 3.9 µg/ml alone or simultaneously with antitumor drugs (doxorubicin, cisplatin, taxol, and tiazofurin; IC50 concentrations) for 2 hours. Growth inhibition was evaluated by colorimetric SRB assay after recovery period of 24, 48, and 96 hours. The genotoxic examination was performed using sister chromatid exchange test and micronucleus assay, at fullerenol concentration ranging from 1 to 5 µg/ml. The fullerenol alone mildly inhibits the growth of both cell lines. Simultaneous administration of fullerenol and antitumor drugs strongly suppressed antitumor drug-induced cytotoxicity. The rate of cytotoxicity inhibition depended on fullerenol concentration, type of antitumor drug and cell line. Protection against doxorubicin and cisplatin was more pronounced than against taxol and tiazofurin. Fullerenol was not found to be genotoxic to investigated cell lines.