To investigate the interaction between anticancer drug resistance and radioresistance in cervical cancer cells, 3 single cell-derived cyclophosphamide-resistant subclones were established from the drug- and radiosensitive human cervical squamous cell carcinoma cell line ME180 by chronic exposure cultures with 4-hydroxy-cyclophosphamide followed by limiting dilution. The established cyclophosphamide-resistant subclones were also radio- and multidrug-resistant to 7 other anticancer drugs. Flow cytometric analysis revealed significantly increased levels of CD40 expression on the 3 resistant subclones, whereas no CD40 expression was found on the parent ME180 cells. However, there were no changes in the expression levels of CD29, CD49a-CD49f or CD59 between the parent cells and resistant subclones. A recombinant human soluble CD40 ligand had no effect on the proliferation of the resistant subclones. Irradiation had no effect on the 4-hydroxy-cyclophosphamide sensitivity of the parent cells. These results indicate that the established cyclophosphamide-resistant subclones have impaired cell death signals, which are common to both drug- and radiation-induced apoptosis, and cyclophosphamide may not be an adequate drug for use in concurrent chemoradiotherapy. Furthermore, CD40 activation signals may be associated with the multidrug- and radioresistance in these cyclophosphamide-resistant subclones.